-Britain's royal palaces... historic... -This amazing space is the grandest medieval hall in the world.

-...extravagant... -It's a magnificent fortress.

But what it also is, is the world's biggest jewelry box.

-...and jam-packed with secrets.

-Sandringham is the royal party palace.

They can do what they want there when they want, and the paparazzi can't photograph them.

-In this series, we gain privileged access inside palace walls... -Old plans suggested that, if they could open this up, they might be able to reveal a long-lost secret.

-...and uncover the hidden treasures within.

-We're opening boxes to find jewels with handwritten notes from members of the family inside.

-We unearth the palace's dark secret histories.

-Underneath this beautiful palace was a secret, bubbling laboratory of horrors.

-And we reveal the untold truth behind the palace's most dramatic modern moments.

-For Harry to be made a suspect for a criminal act is really, really serious.

[ Gunshots fire ] -I saw a gunman actually holding a gun and pointing it straight at the Queen.

-We kept it a secret, but it surprised a whole audience and quite frankly, the world.

-This is the all-new "Secrets of the Royal Palaces."

-Stop them!

-Off with their heads!

-This time, Charles and Camilla come under attack after leaving Clarence House... -That split second, you could have been looking at an incident that would have changed the course of history.

-...Harry and Meghan's secret love nest at Kensington Palace is rumbled... -It's where he began to conduct this romance with Meghan, and for a while, no one knew she was there.

-...we reveal the secret remains of Greenwich Palace... -Down these stairs is the only surviving intact space from the Tudor Palace.

-...and uncover why Buckingham Palace's fanciest coach is so despised by its royal passengers.

-King William IV said it was like being tossed around in a stormy sea, and Queen Elizabeth II admitted later that it was indeed horrible.

-Today's royal palaces are armed fortresses, protecting the family from any kind of attack.

-The safest place for any member of the royal family is within the palace.

-We have high walls.

We have fences.

We have armed police officers with guns.

We have various technical security measures which will be alarms, sensors.

All these link into a set of procedures.

-It's when the royals venture outside palace walls that the protection officers get twitchy.

-Royal protection officers recce -- reconnaissance -- every route, every location, every engagement that a royal makes.

And nothing is left to chance.

-In a convoy scenario, you're working with the special escort group, the group of motorcyclists that are specially trained to move you through the crowd so that you aren't stationary and therefore have the possibility of becoming a target.

-In December 2010, Charles and Camilla were setting off on a routine trip to the Royal Variety Performance.

-The protected convoy was moving from Clarence House all the way to the London Palladium through the streets of Mayfair.

On that day, there had been large-scale public disorder in relation to the increase in student-tuition fees.

-What the security team hadn't factored in when they left the Royal Palace was just how unpredictable the protesters had become.

-Small groups of protesters split off from the main and started kind of rampaging through central London.

-On the scene was photographer Cliff Hyde.

-The biggest group started heading north up Regent Street.

They were smashing windows, kicking over bins, and they turned round, and the highly recognizable dark-red Rolls-Royce had been driven into the back of the crowd.

[ Siren wailing ] -They were physically blocking the car from going anywhere.

They were completely surrounded by a hostile crowd.

-You've got the heir to the throne and his wife, absolutely dripping in jewels, in a car worth several millions of pounds, going through what was effectively a riot.

Massive potential for disaster.

Inevitably, you're going to get militant groups, who go to these things looking for a fight.

And they found Charles and Camilla.

-When the crowd first surrounded the car, they were both still kind of waving and smiling.

-Oh, Charles, how you doing, mate?

-Then the reactions changed.

This is so unprecedented that they probably can't believe themselves what's going on.

And for about 30 seconds, the mask slips a little bit and you can kind of see what's going on in their heads, and it's fear.

-Let's go!

Off with their heads!

Off with their heads!

-And you can hear, "Off with their heads!

Off with their heads!"

You know, referencing the French Revolution.

-As protesters kicked and rocked the car, Cliff was there to capture it all on camera.

-Tory scum!

-The photographs that were taken of his and Camilla's faces told the whole story.

They were shocked.

They were frightened.

He was very worried for his wife, and they're sort of recoiling from kicks being aimed at their car.

-I'm intentionally staying back, because I know the guy in the front passenger seat has got a gun, so I wanted to make sure I was not considered a threat and stayed well back.

-At that split second, you could have been looking at an incident that would have changed the course of history.

-If this happened in America, the police would have just opened fire on the crowd and many people would have died.

-Luckily, the police soon intervened, but it could have been a very different story.

-Some public order units managed to catch up with the demonstrators, and they cleared a path for the protected convoy to go through.

-Charles and Camilla finally arrived at the Palladium, shaken but unharmed.

-They got out at the Royal Variety Performance, met the Lord-Lieutenant of London, shook everyone's hand, sailed serenely into the theater, and enjoyed the show.

Total pros.

-But the state of their car told the story of their ordeal.

-This beautiful, vintage Rolls-Royce -- dents in all the bodywork, I think a window had been smashed.

It was really, really trashed.

It had a huge dollop of paint on the back end of it, where someone had obviously chucked something at it, 'cause they didn't have an egg to hand, presumably.

-Perhaps next time there's a protest, the royals will decide to have a palace night in.

Nine miles to the east of Clarence House lies the site of one of Britain's greatest lost palaces -- Greenwich, birthplace and sometime home to England's most notorious monarch.

-This was the place where Henry VIII's Greenwich Palace stood, the place that witnessed his emergence into this world.

It's long gone, replaced by the old Royal Naval College.

-Henry's great palace fell into disrepair after his death, and nearly all traces of this historic building were lost.

But there is still one remaining part if you know where to look.

-Down these stairs is the only surviving intact space from the Tudor Palace, a secret space from the age of Henry VIII.

♪♪ This is the undercroft of part of the Great Hall of the Palace that was built in the years around 1500 by Henry VIII's father, Henry VII -- a place to store wine for the major rooms above, which were used for dining.

You can imagine the barrels of wine filling this space, kept in good condition, because this is subterranean, with a solid masonry vault over the top.

These ribs are built of brick, once covered in plaster to look like stone, and they sit on columns -- very finely molded stonework.

It doesn't have to be like this, but it's just beautifully constructed.

♪♪ -These are the last physical remains of a site of nation-changing royal events.

-So not only did Henry VIII marry two of his queens at Greenwich, but two of his children were born there.

Mary I and Elizabeth I were both born at Greenwich.

-Greenwich Racecourse, also the site of the very infamous episode with Anne Boleyn, where she was seen to drop a handkerchief at the Tiltyard, accused of treachery and treason, transported down the Thames to the Tower of London, and she was beheaded.

♪♪ -But it wasn't just deaths and births.

Greenwich was also home to Tudor chivalry.

-This is the site, rumor has it, where Sir Walter Raleigh lay down his cape in a plashy place for Elizabeth I to cross over so that she kept her feet dry.

What an act of gallantry!

But it's first recorded 80 years after the event.

Now, is it history, or is it legend?

-Whatever the truth, the Tudors were synonymous with Old Greenwich, but it was a more obscure queen who shaped the site we see today, as we'll reveal later.

Coming up, the race is on to capture a royal romance.

-Sources had told me that Meghan was coming over to stay with Harry at Kensington Palace.

-And why the King had to move out of Hampton Court.

-It was essential, because if he didn't, the human poo would mount up to a massive mountain.

-Today, Buckingham Palace is the public face of the royal palaces, a showcase both outside and in.

Its 600 rooms are packed with priceless art, statues, and jewels.

But one of its greatest treasures can be found hiding 500 meters away in the Royal Mews.

It's the royals' glitziest set of wheels.

-The Golden State Coach is about as unsubtle as you can get if you want to make a massive entrance.

It's straight out of a fairy tale.

You can imagine Cinderella turning up in it, having it been transformed from a pumpkin.

-The whole thing is covered in gold leaf.

It is astonishingly bling.

-The coach was originally commissioned for King George III in 1760, to transport him from Buckingham Palace on special occasions.

-At the time, it cost £7,562, which today is equivalent to about £1.5 million.

-It has been used for every single coronation since George IV.

Most famously, it was used for the coronation of Queen Elizabeth II in 1953, and she traveled down the mall in the Gold State Coach looking down to the crowds, who were all cheering around her.

-This sensational gold coach is adorned with secret royal symbols.

-On the very top are three huge cherubs that represent England, Ireland, and Scotland.

And they hold the orb, the scepter, and the badge of knighthood.

-And then best of all, we have these four Triton figures -- half men, half fish -- who are moving forward with the coach, blowing on their conch shells, their hair, their mustaches flying back in the wind.

It is just the most extraordinary symbol of naval power, of British victory at war.

-It is the ultimate expression of luxury.

But it hides a secret problem.

-It is incredibly uncomfortable to ride in.

-King William IV said it was like being tossed around in a stormy sea.

Queen Victoria, well, basically she refused to ride in it, and Queen Elizabeth II said that it was a little uncomfortable.

And then I think she admitted later that it was indeed horrible.

[ Crowd cheering ] -One of the reasons that the coach is so very uncomfortable is its size.

It is ginormous.

It's 7.5 meters long, almost four meters high.

-And the coach is four tons in weight.

It is so heavy that the thing needs to be pulled by eight huge Windsor Gray horses, who can only move it at walking pace.

So the poor royals who are sitting inside -- their torture is drawn out for as long a time as possible.

-We have to remember, this coach is 300 years old, and at the time there weren't major developments in things like suspension.

The Diamond Jubilee State Coach -- that has hydraulic suspension, it has a heating system, electric windows.

But the old coach has absolutely none of those comforts.

-This might explain why the magnificent Gold State Coach so rarely leaves the Royal Mews.

-The Gold State Coach hasn't been on the streets of London for 20 years now, but it did make one very special appearance during the Queen's Platinum Jubilee.

It was taken through the streets of London, up the mall, pulled by those eight Windsor Grays.

-But the Queen herself chose not to ride in it and instead chose a hologram to appear in her place.

♪♪ -From shiny coaches to sparkling chandeliers, today's royal palaces are pristine and beautiful, but that hasn't always been the case in Tudor times.

Hampton Court Palace was head to toe in filth.

♪♪ -Henry VIII -- we often think of him as rather a grimy, greedy king.

That wasn't the case.

He was incredibly clean.

He washed every couple of days.

He always wore clean linen.

He actually wore a piece of fur next to his skin to stop the mites from getting at him.

And when it came to his palaces, he decided to launch a war on dirt.

For example, he brings in some urinals around Hampton Court Palace so that the courtiers can use them.

He actually also builds a giant mass latrine -- 14 people to use.

The idea that you go to the loo here, not in the corridor.

He also marked up the walls with X's -- Holy Crosses to try and dissuade people from weeing against the walls, because the idea was that it's a holy spot.

Unfortunately, there's not only nowhere for the urine to go, there's nowhere for the other stuff to go.

Underneath the palace is a giant chamber where all the human excrement is kept.

So Henry brought in a lot of cleanliness measures, but it just didn't work.

In the end, the only way to escape the filth was to move.

They go to one palace, then they leave it, give it some time to air out and some of the dirt to fade and the smells to fade.

But also, this is the only moment that you could move out from below the chambers, the epic mound of human excrement, and bury it elsewhere, so really, it was essential to move palaces, because above all, if you didn't, the human poo would mount up to a massive mountain.

-Outside their palaces, our royals are always in the public eye.

It's behind the gilded gates where they can enjoy a private life, free to live, relax, and even fall in love, away from the media's prying eyes.

-No matter how long your paparazzo photo lenses, you're not gonna get a photo of a newly loved-up couple inside a royal palace.

-As far as any media are concerned, anyone who's in that palace has a reasonable expectation of privacy, and so you're not allowed to, even if you could.

-When Prince Harry started dating the American actress Meghan Markle in July 2016, he wanted to keep it hidden from the press.

Luckily, Harry knew just the palace for a secret liaison.

-Harry had inherited from his brother Nottingham cottage which is a standalone little house inside the grounds of Kensington Palace.

And it's where he began to conduct this romance with Megan, and for a while, no one knew she was there.

-The Palace will have helped keep their relationship a secret to some extent because it's extremely secretive, and it's a huge, sprawling estate, and there's lots of ways in and out.

It is possible, if you have a hat on, to wander in and out and almost pass as one of the members of staff, and when you come out, you're in very busy Kensington High Street.

You just look like a tourist, who's wandering about the gardens.

-Quite quickly, Meghan was given her own security pass so that she could come and go as she pleased, and could wander Kensington High Street.

And at that point, no one knew the two of them were dating.

It's very unusual for a girlfriend to be staying very regularly behind the scenes with a royal on royal property, but there was extreme circumstances at play here.

Prince Harry is one of the most famous young men in the world.

Whenever he's seen with any female, the world would just go crazy.

The only way they could pursue a normal romance was to do it all behind closed doors and on royal property where they wouldn't be seen.

-He'd had two previous high-profile romances -- one with an actress Cressida Bonas and another with Chelsy Davy, a businessman's daughter, constantly conducted in the glare of the public spotlight.

And he was determined with Meghan to give it a chance.

I mean, those romances with those two other girls largely broke down because they didn't enjoy the public spectacle of being seen on the arm of a high-profile royal.

-He was genuinely worried he would never find someone who would put up with all of the paraphernalia and the publicity and the scrutiny.

-For several months, Harry managed to keep his new girlfriend hidden from the press, but very few royal romances can be kept under wraps for long.

After four months of secret dating, they were outed by the "Sunday Express."

-Harry was furious that the news had got out.

Every other newspaper was furious that they had missed the scoop.

Next big thing was getting the first photograph of them together.

-Harry's battle with the press was on.

-There was some suggestion that maybe they should go to the rugby match together, be pictured there, and that would then kill the market for the picture.

But Harry did not want to play that game.

He was so determined to protect her.

He wasn't gonna do any sort of sweetheart deals with a particular magazine or whatever for a picture at an event.

He was gonna announce it on his terms when he wanted to.

-Harry is instinctively a secretive man.

He wanted to keep the romance as private as he could.

-But Harry knew the paps would do everything they could to get their hands on the photo.

-Paparazzi, who operate just on the basis of being paid for one particular good shot, would have known there was a huge payday coming if they could get it.

Staff photographers and royal photographers, who work directly for newspapers, would also have had a lot of pressure on them from their editors to try to be the ones who got the shot.

-In December 2016, Harry arranged for Meghan to spend Christmas with him, hidden at Kensington Palace.

It was meant to be a secret, until word of the visit leaked to "The Sun's" royal editor, Emily Andrews.

-Sources had told me that Meghan was coming over to stay with Harry at Kensington Palace, so I was very excited.

I thought, "[Gasps] Could this be the moment where we could maybe get a photograph of them together?"

-Coming up, Harry and Meghan take on the paps circling Kensington Palace... -I think the photographers were a bit nervous about upsetting Harry.

He's quite a volatile character.

-...and how a mini palace in Greenwich revolutionized our towns and cities.

-That is one of the most important examples of architectural history in the whole of Britain.

-In 2016, Kensington Palace was harboring a secret romance between Prince Harry and Meghan Markle.

After four months of clandestine dating, they were outed by the press.

But Harry was determined to keep as much of their relationship a secret as possible.

-Harry had issued an extraordinary statement.

He'd more or less asked -- told the media to back off, that the they were in the early stages of a romance.

-But the press refused to back off.

-The media knew that she was in London, staying with Harry at his Kensington Palace cottage, and of course, everyone wanted to get the first picture.

-They were away from prying eyes there, so it was only when they left the palace gates that they might be spotted.

-Not wanting to become prisoners in their own home, Harry arranged to take Meghan on a date.

-Meghan is an actress.

What does Harry do?

He takes her to one of the famous theaters on Shaftesbury Avenue, in the heart of London's Theatreland.

-But news of the date was leaked to "The Sun's" royal editor, Emily Andrews.

-So I sent three of the best photographers -- one outside Kensington Palace and then two to Soho to wait in Theatreland.

-And so began a game of cat and mouse, familiar to both Harry and the press.

-There are pretty strict rules about what you can and can't do with photographers with royals.

You can't follow.

You can't track.

You can't do anything like that.

-To mislead the paps, the couple secretly left the palace incognito.

-They didn't have the blues and twos.

They didn't have any police outriders.

It was just them in one vehicle.

Blacked out windows, not a posh car.

The first photographer at Kensington Palace saw a car leave but wasn't sure it was Harry and Meghan, didn't get a picture, and said it was heading east.

-Harry had got through and was on his way to the West End.

-What would often happen -- Harry and Meghan would wait until the lights were down, and then they'd slip in and take their seats and try and be as incognito as possible.

And that evening, they got stuck in traffic on Regent Street, and because they were running late, they decided to get out of the car and walk to the theater.

And it was then that the photographer spotted them.

-But if Harry clocked their camera, it would be game over for the paps.

-Harry had always had his eyes scanning the horizon for photographers.

He's very much like his mother, Diana, who had this ambiguous relationship with the media and the photographers in particular throughout her life.

-I think the photographers were a bit nervous about upsetting Harry.

He's quite a volatile character.

He's got a lot of very expensive lawyers on speed dial, and he's got the police on speed dial.

-With Harry and Meghan minutes away from the theater, the photographers needed to use secret tricks of the trade.

-One of the photographers spotted Harry and Meghan walking down Regent Street, used a bus shelter almost as, like, a protection, and shot the pictures through the bus shelter.

[ Camera shutter clicks ] That's part of the appeal of the photograph.

It wasn't some staged, managed photo call.

This was a very candid photograph.

I really love the fact that the first pictures we had were very low key, they were very in sync with each other.

You know, the matching beanies was kind of giving the kind of "We're in it together" vibes, you know, holding hands.

-There was something about that photograph that instantly relayed to the world that this was no flash in the pan, this was no passing romance.

This was a couple who were very serious about one another.

-The first time that Harry saw the photograph was on "The Sun's" website that night.

[ Camera shutter clicks ] -Harry rang up his press secretary, raging down the phone, who then rang me, and I said, "Well, I'm sorry.

Paper's gone to press.

Nothing you can do about it."

-This photograph would set the tone for Harry and Meghan's relationship with the press and the palace.

-The fact that the first pictures of them that emerged were paparazzi shots rather than something staged absolutely says that Harry is going to do his own thing.

He wasn't going to play up to the press.

He wasn't going to give them what they wanted.

-I think that first photograph was the beginning of that battle with the press for Harry and Meghan that culminated in Megxit and culminated in them leaving.

And I'm quite sad about that, actually.

I think they had so much to offer, and I think it's very sad that they left.

♪♪ -From Harry and Meghan to Charles and Camilla, the royals are Britain's most famous family and their palaces are just as recognizable.

-Buckingham Palace is undoubtedly the best-known residence in the world.

It's right up there with the white House.

-But some former royal residences have fallen into the shadows.

Tucked away in the leafy royal borough of Greenwich lies one of the most important buildings in the land, built for Queen Anne of Denmark, wife to James I, allegedly following a bizarre hunting accident.

-The story goes that Anne accidentally shot one of James I's favorite hunting dogs.

James abused Anne and then repented, and as part of his remorse, he had the Queen's palace especially built for her.

-Her husband supplied the money, but it was Queen Anne who commissioned a cutting-edge architect to design a home like nowhere else in the country.

-That is one of the most important examples of architectural history in the whole of Britain.

The Queen's House at Greenwich was designed by Inigo Jones, who had been a designer of stage sets for King James I.

-Inigo's revolutionary mini palace would be Britain's first classical building since the Romans.

-Classical design is all about symmetry and proportion, and this building has a strict symmetry.

One side is just like the other one in a mirror image.

The straight lines of the balustrades and the sharp edges of the walls are all about describing the tight proportions that organize this building.

It's a jewel box, and it would have been in stark contrast to the sprawling red-brick palace that was set against the banks of the Thames behind it.

It was wildly influential.

In the next 200 years, every town and city in Britain was in some way influenced by this one building.

-Unfortunately, Anne didn't live to see the building completed, which was only finally completed several years later and overseen by Henrietta Maria, the wife of Charles I.

-Built between the grounds of the old Tudor Palace and the park beyond, the Queen's House is, in fact, one of the grandest bridges in England.

-You'd never know it today, but this was once the main Woolwich to Deptford road, and Inigo Jones cleverly designed the Queen's House to straddle it.

These bridges gave the Queen a private route from the house to the park, so she didn't have to mix with the carts and the riff-raff.

-No expense was spared, and the inside of the Queen's house was designed to be as shockingly modern and dazzling as the exterior.

-This is the centerpiece of the design, the Great Hall.

Inigo Jones designed it with a pure geometry.

With symmetrically arrayed Italian details, the whole thing an import from Renaissance Italy, and England had never seen anything like it.

This was a new vision in England, a land used to parades of random tapestry-lined halls.

On the floor, there's a geometric array of circles and squares, and that's mirrored on the ceiling.

-And hidden inside is a radical piece of engineering.

-This isn't any flight of stairs.

This is the Tulip Stairs.

And it's remarkable because it's a self-supporting spiral, where each step emerges from the wall to support the one above it, relieving the center of any structure.

It's the first of its kind in England, which must have been incredibly modern and impressive.

-But its name is misleading.

-The name "Tulip Staircase" refers to the flowers in the wrought-iron railings, but that name hides a secret, because those flowers aren't tulips.

They're actually lilies, which are the royal flower of France, to complement Henrietta Maria, who was born in France.

-The staircase is an outstanding survivor of Greenwich's royal past.

♪♪ But survival can be hard at some royal palaces.

As a rule of thumb, it's always best to obey the monarch, especially if you're a maid of honor to Queen Elizabeth I.

♪♪ -It's 1560, the court of the Virgin Queen, surrounded by her ladies, who were also virgins, too.

Or so she thought.

There was a complete, shocking scandal at the heart of the royal court, and it was over Katherine Gray.

Now, Katherine Gray is a real threat to Elizabeth.

She's her cousin, and therefore she could be an heir to the throne.

And what Elizabeth doesn't want is Katherine Gray getting married, because this will strengthen her, and also Katherine Gray having a son, because that could be a complete challenge to Elizabeth.

But behind Elizabeth's back, Katherine Gray has fallen desperately in love with a young man called Ned Seymour.

It's terribly romantic.

They get married in secret.

Now they're married.

They basically have sex in all of Elizabeth's palaces, everywhere.

They have it in Greenwich, in Westminster.

You name the palace, they're having sex in it.

And the inevitable happens.

Katherine falls pregnant.

This is a big problem.

She cannot hide a pregnancy from the Queen.

Elizabeth is furious.

She blows her top.

This is an incredible threat to her.

Katherine Gray has got married without her permission, she's really disobeyed Elizabeth, and also on top of that, she's pregnant.

So she throws Katherine Gray into the tower, and when Ned gets back from his trip to Europe, she throws him in the tower as well.

Katherine gives birth to her son in the tower.

And then, well, the tower isn't quite what it's supposed to be, because Katherine and Ned's rooms are ten feet away from each other, and an obliging jailer says he'll open the door so that Ned can slip in to Katherine's room.

They make merry on Katherine's bed, and Katherine gets pregnant again in the tower, and this is when Elizabeth goes completely mad.

Elizabeth claims the marriage is not real.

The marriage annulled.

Any children will be illegitimate.

They are not a threat to her.

Elizabeth says that Katherine and Ned must be separated and that Katherine must stay with male relatives -- pretty much keeping her under house arrest -- also must be separated from her eldest son.

Katherine is heartbroken.

She wants her husband, she wants her son, she's completely devastated, and she pretty much wastes away of a broken heart and dies aged 27.

And after this, Elizabeth keeps a very close eye on what her ladies are up to in the palaces.

-Still to come, the royal building at Greenwich is used to get one up against an old enemy.

-What it shows is William and Mary crushing the Catholic rival, Louis XIV of France.

-And murder most horrid at St. James's Palace.

-His body is covered with these awful pustules.

There's this terrible smell.

His back is brown.

♪♪ ♪♪ -Royal palaces don't last forever and can very quickly become less than palatial.

After the English Civil War, the Tudor Greenwich Palace was used as a biscuit factory and prisoner of war camp before eventually being demolished in 1660, but the land remained in royal hands, and in 1694, Queen Mary II ordered a new architectural gem for the Greenwich landscape.

-Mary the second, seeing injured sailors coming back from the Nine Years' War, felt inspired to commission a hospital for seamen at Greenwich so that they could recover and convalesce there.

It was designed for free by Christopher Wren, who we might associate with Saint Paul's Cathedral.

-Today's skyline of Greenwich contains a secret that dates back to one of Mary's demands when she commissioned the hospital.

-Queen Mary ensured that the hospital buildings were split to provide an avenue that led from the river, through the hospital grounds, up to the Queen's House, and then Greenwich Hill beyond.

That placed the Queen's House central in the ground plan, and it still provides the basis of the Greenwich skyline today, although the Queen's view, protected by her, is now focused on Canary Wharf.

♪♪ -This magnificent royal ensemble is now recognized as a World Heritage Site, but arguably the most extraordinary masterpiece of all lies hidden inside.

♪♪ -In 1707, the 32-year-old painter James Thornhill won a commission to create a jaw-dropping tribute to England's Protestant monarchy.

The Painted Hall is sometimes known as Britain's Sistine Chapel, and you can see why.

There's 40,000 square feet of revealing imagery on its walls and ceilings.

-This astonishing mural was painted at a time of religious division in Europe between Protestant and Catholic, and its striking images contain hidden and not so hidden messages about the faith of Britain's rulers.

-This is a giant piece of propaganda for the Protestant monarchy.

And what it shows is William and Mary as Protestant monarchs crushing the Catholic rival Louis XIV of France, with Louis fallen at their feet, spilling his crown.

And they're framed by the classical gods, offering them their approval.

It's a great rivalry, and that goes hand in hand with the idea that Protestantism trumps Catholicism.

-Thornhill spent 19 years planning and painting his masterpiece.

-The room was initially intended to be a dining hall for the sailors, but by the time it was finished at astronomical expense, it was deemed way too fancy, and so they had the dining room downstairs.

This, in the end, became a showpiece.

It's an astonishing work of art, but it really cricks your neck.

The best way to take a look at it is to take a lie down.

♪♪ -For all their dazzling beauty, a royal palace can be a dangerous place, full of jealous rivals plotting secret schemes, especially for courtiers visiting St. James's Palace in the 17th century.

♪♪ -Palace intrigues can get pretty brutal, especially when everyone is circling around the king to try and get favoritism.

And James I had a lot of male favorites.

One of his great favorites was Robert Carr.

And now Robert Carr was not the sharpest knife in the 16th-century drawer.

So he became really very dependent on his good friend Thomas Overbury, who was a much more intellectual man.

So when the King asked Robert Carr to do something for him, Robert Carr would then really ask his friend Thomas to do it.

Thomas and Robert had this bromance.

They were very close.

They really were sharing power in a way.

But they all fell apart when Robert fell in love with Frances Howard.

Now, Thomas Overbury did not like her.

Because she was part of the powerful Howard family, he immediately saw that his position as chief friend of Robert was going to be over when he gets married, so he begs Thomas -- says, "Don't marry her."

But Robert is in love with Frances, so it becomes a battle between Thomas Overbury and Frances Howard.

And really, she wins.

She maneuvers.

She gets Thomas out of court.

She also gets him into prison.

You think that will be enough, but then Thomas dies in the tower -- a terrible, awful death.

His body is covered with these awful pustules.

There's this terrible smell.

His back is brown.

And really, it all looks very suspicious.

So they exhume Thomas Overbury, they do an autopsy, and find that he was actually poisoned.

They then investigate, and the trail goes to Robert Carr and Lady Frances.

And finally Lady Frances confesses what she did was she brought in a jailer to the tower that was friendly to her, who knew about drugs, and he, along with the wife of a physician, they ply poor Thomas Overbury with sulfuric acid, so he dies.

Robert and Frances are sentenced to death and locked in the tower, but just a few years later, they're pardoned and set free, so perhaps the king forgave his old favorite after all.

♪♪ -Next time, behind the scenes at a Buckingham Palace party that almost went up in flames.

-But apparently it's treasonable.

We'd have all been taken to the tower.

-We discover the diamond ring that started a palace love story... -There is a secret to this ring.

It was a private message from Prince Philip to Princess Elizabeth.

-...and a deadly cover-up at Sandringham.

-How shocking is that?

The British king was killed in his own bed, in secret in his own palace.

♪♪ ♪♪ ♪♪ ♪♪