DAN JONES: For me a great British castle is a fortress, a palace, a home.

And a symbol of power, majesty and fear.

For nearly 1,000 years castles have shaped Britain's famous landscape.

These magnificent buildings have been home to some of the greatest heroes and villains in our national history.

And many of them still stand proudly today, bursting with incredible stories of warfare, -treachery, intrigue and even murder.

-(SWORDS CLANKING) Join me, Dan Jones, as I uncover the secrets behind six great British castles.

This time, I'm in the southeast of England to explore the history of Britain's greatest defensive castle.

This imposing fortress on the white cliffs of Dover has resisted all enemies from the Middle Ages right up to the Cold War.

(SEAGULLS QUACKING) The White Horse Pub, tucked away on the back streets of Dover is plastered in the names of people who've stopped in here for a pint.

Not that the landlord seems to mind.

This pub encourages graffiti, but only from a select group of people.

But if you can discover what connects all these scribbles on the walls, then you'll understand the key to Dover and to its castle.

That key is just a stone's throw away.

The Dover Strait.

Or as it's usually called, the English Channel.

All of these people are successful cross-channel swimmers.

And they knew the same thing that the Romans knew and the Saxons knew, that the Medieval French knew, that Napoleon and Hitler knew.

That the quickest way across the Channel from mainland Europe to mainland England was through Dover.

From Dover, it's just 21 miles to France.

If you're quick, you can swim it in 10 hours or so.

It's even quicker on a boat, and that's why it's dangerous.

If all of England is a castle, Dover is the main gate.

The ancient Britons' word for water was "dubra" or Dover.

This was the edge of the kingdom beyond which lay the rest of the known world.

So any invasion, any attack, had to come through Dover.

Dover was and is the key to England and that's why it had to be protected by one of the greatest castles in Britain.

It has towers, gatehouses and a curtain wall that's a mile long.

And in the middle of it all stands the formidable inner bailey and massive stone keep.

In its time, this gigantic fortress has kept Britain safe from the Medieval French, the Emperor Napoleon and Adolf Hitler.

But the story of Dover Castle starts long before any of that.

Dover's strategic importance began with the Romans.

In the 1st century BC, this is where Julius Caesar and his legions first landed in Britain.

And here, on Dover's eastern heights, the Romans built an incredible structure that still survives nearly 2,000 years on.

It's called the Pharos, a great lighthouse designed to guide Roman galleys to the shore.

When William the Conqueror invaded England and ransacked the Saxon fort, he was quick to realize just how important Dover was and he ordered it rebuilt as soon as possible.

The credit for the massive stone fortress you can see here today has to go to William's great grandson Henry II.

Henry II ruled England for more than 30 years at the end of the 12th century.

And he laid many of the foundations for the country as it is today.

Henry II built what is still the centerpiece of Dover Castle.

A great bailey wall with 14 defensive towers protecting one of the most impressive medieval keeps ever constructed, The Great Tower.

In his day, Henry II was the big dog among the kings of Medieval Europe, and he loved building castles as statements of his royal authority.

But Dover Castle, well, that has to be the ultimate symbol of royal supremacy and power.

But the magnificence and scale of Dover Castle that we see today exists because of Henry's embarrassment and guilt.

Dover Castle was a fortress, but it was also a palace on which no expense had been spared.

Inside were two palatial suites of rooms, suitable for putting up important guests as well as the king himself.

But on the second floor, a narrow corridor leads away from the impressive staterooms.

At the end of it is a room of special significance to the king.

A place of reflection and regret.

This chapel I'm sitting in now was built by Henry II himself.

It was built to venerate a saint.

But not an ordinary saint.

This was a man that Henry had personally caused to be killed, to be murdered in the most brutal way imaginable.

His name was Thomas Becket.

Becket was once Henry's best friend.

He was his fixer and his closest counselor.

But when Henry made Becket Archbishop of Canterbury, their friendship blew up in spectacular fashion.

Henry wanted Becket to help him impose royal law on the English Church.

When Becket refused, the two became bitter enemies.

...in nomine Patris, et Filii, et Spiritus Sancti... Ave Maria, gratia plena... JONES: In late December, 1170, four of Henry's knights rode to Canterbury with murderous intent.

Amen.

...ventris tui Iesu... JONES: They found Becket unarmed in his cathedral.

They cut him down near the altar and mashed his brains into the floor.

It was an atrocity that shook 12th-century Europe.

Two years later, Becket was a saint, but Henry was a hate figure.

To restore his damaged reputation, the king performed an extraordinary act of penance Henry turned up at Canterbury, barefoot and dressed in sackcloth robes.

Then, he ordered the cathedral's 80 monks to beat him three times each with a wooden rod.

The king then spent the night alone at Becket's tomb praying for forgiveness.

From this point on, Becket's shrine became Europe's number one tourist destination visited by pilgrims from all over the continent.

So in a sense, this chapel, and indeed the whole castle, was founded on Henry II's guilt.

Among the many visitors to Becket's tomb were the great and the good of Europe, and in 1179, Henry received word that one of the greatest of them all was coming to visit.

Louis VII, King of France, who was coming to Becket's tomb to pray for his seriously ill son.

The King of France was coming.

This was the first state visit in English history and Henry had to put on some kind of grand show.

No French King had ever set foot on English soil before.

But Henry had a problem.

At this stage, Dover was a long way from being a suitable place to receive royalty.

Now, this was slightly embarrassing.

Henry actually had to meet the King of France on the beach before riding with him all the way to Canterbury.

And perhaps because of that humiliation, within a couple of months, Henry had started throwing money at the development of Dover Castle.

In the last 10 years of his reign, Henry spent more on Dover than on any other English castle.

He did it to make sure his legacy would never be overshadowed by the murder of Thomas Becket.

Dover Castle would remind every pilgrim that passed this way, the wealth and authority of a great king.

But Henry also made sure to build Dover as a proper military fortress and it's just as well that he did.

Dover Castle was the largest and most strategically important English fortress of the Middle Ages.

King Henry II, who founded the castle, would never see it tested in war.

But his son would.

At the turn of the 12th century, these walls would face their first great challenge under the command of England's most despised ruler.

A monarch with one of the worst reputations in all of history.

The infamous King John.

Everyone knows King John from the Robin Hood stories, but actually, his role in British history is much more important than that.

He nearly destroyed Dover Castle and the whole kingdom.

To find out why, I'm headed to Salisbury Cathedral in Wiltshire, 160 miles west of Dover.

Here, in the magnificent ChapterHouse, is an extraordinary manuscript, a document that has resounded throughout history.

Its name is Magna Carta.

This is one of four surviving copies of Magna Carta, the treaty drawn up in 1215, 800 years ago, which is still one of the most famous documents in the whole of Western history.

And today we think of Magna Carta as underpinning the rule of law, justice, human rights, English liberties and even the US Constitution.

But in fact, in 1215, it was a peace treaty between King John and his rebellious barons.

John was a cruel and vengeful king.

He imposed crippling taxes to fund unsuccessful military campaigns.

He alienated his subjects, plundered the Church and waged war on his barons when they finally rose against him.

Magna Carta was a desperate attempt to bring the king to heel.

There are about 4,000 words in Magna Carta, and we usually divide them up into 63 clauses.

And if you read most of it today it doesn't seem very relevant.

It's stuff about tax rates, and fish weirs and bailiwicks, and all things basically forgotten in England today.

What was more important was clause 61, right at the bottom, which we now call the security clause.

And what that said, was that if King John broke any of the terms of Magna Carta a council of 25 barons is legally entitled to make war on him.

And in 1215, that's actually exactly what happened.

John appealed to the Pope, claiming that by restricting the rights of the King, Magna Carta was an attack on the authority of the Church itself.

The Pope agreed.

The rebel barons were excommunicated and England erupted into civil war.

The barons knew they could never make peace with King John, so they invited Prince Louis of France to invade England and take the throne from him.

Inviting a French prince to become king of England might sound pretty treacherous, but actually the English were pretty used to it.

In fact, it had happened three times in the 150 years since the Norman Conquest.

A French baron had come across the sea, hit the south coast, taken the crown and put up massive castles to enforce his power, just like the one at Dover.

Within weeks, Louis and the rebel barons had control of Canterbury, Rochester, Winchester and London.

England was falling to him, but what he really needed was the crucial communications center between England and France, Dover Castle.

The fate of King John and of his realm now rested with Dover.

Louis immediately laid siege.

Arranging his forces on a hill to the north of the castle, he battered the walls with catapults.

But these defenses held firm.

So Louis changed his strategy, exploiting the very thing that made Dover famous.

The white chalk cliffs beneath its foundations.

Instead of going over the walls, the French would go under them.

Beneath the walls of Dover Castle, French engineers began tunneling through the rock trying to weaken the foundations, a siege technology known as undermining.

JONES: Undermining, what are we talking about?

PAUL PATTISON: Well, as you can see here, the chalk is the rock... -Mmm-hmm.

-...so they have to tunnel through it underneath the castle foundations to try and make them collapse.

So if the wall collapses and the gate collapses, you end up with a rubble slope, up which the French attackers can clamor and get into the rest of the castle.

And how do you stop them?

Well, one of the methods that you use is to dig your own tunnels.

And this tunnel that we're standing close to might just be one of those.

JONES: What would that have been like?

Would there have been hand-to-hand fighting in the tunnels?

PATTISON: If you discover a French tunnel coming towards you and you break into it, yeah, they would have had a scrap in here, and it would have been pretty naughty because it would have been in the dark.

They succeeded in undermining one of the towers at the gatehouse and brought it down, and actually got inside the outer bailey of the castle.

So they got over them.

They got over the defenses.

However, the strength of the English resistance inside forced them back over the breach that they'd made.

And the English made it good with timber and rocks and other temporary measures.

And the French couldn't get in.

They'd lost the impetus.

JONES: On the 14th of October, 1216,frustrated by three long months of siege, Louis negotiated a truce.

Four days later, having contracted dysentery, King John was dead.

Dover Castle's one real weakness had been exposed.

The castle's engineers repaired the undermined north gate, but they also expanded the underground tunnels.

By the time they were done, Dover's defenses would extend way beyond the great curtain wall.

Dover was enlarged and re- fortified by several English kings.

But it would be 600 years before the castle and its tunnels would see action again.

At the end of the 18th century, Britain was at war with France.

All across Europe, Napoleon Bonaparte had crushed his enemies.

And his eye was now firmly fixed on Dover.

By 1803, Napoleon had assembled an army of 130,000 men and 2,000 barges at Boulogne, in northern France.

A massive force just 60 miles from Dover Castle, dedicated to one purpose, the invasion of England.

French invasion was expected on an almost hourly basis, and once again, Dover Castle was at the heart of history.

The most obvious invasion route, Dover.

The French would need a port to resupply their armies, Dover.

The English needed to prepare with might and ingenuity, and they found it in a military engineer called William Twiss.

Twiss set about turning Dover Castle into a modern fortress.

He added five gun emplacements to the outer wall and a huge raised cannon platform at the north gate.

He also reinforced the roof of the Great Tower with brick vaults to support heavy artillery.

But it was below ground that Twiss showed real ingenuity, because Dover Castle had a problem.

There were troops, but there was nowhere to put them.

PATTISON: During the Napoleonic wars, we were expecting an invasion at any time.

The castle was full, already, of troops.

There was no more space above ground, so a character came up with the novel idea to build one underground.

So this is like digging out a basement under your house if you want more room.

How was this dug out?

PATTISON: This was dug out by hand with picks, shovels and barrows.

Hard labor.

So we can see here the wall feels pretty... I mean, my hand's green, it's damp and mossy.

It is damp and it's cold, but to the soldiers of the early 19th century, who are used to pretty awful conditions in the field, it probably wasn't too bad.

And this whole thing, really, is just so ingenious.

So this is just one part under the castle of a much broader network of fortifications for the whole country.

Defending the southeast of England was one of the most important aspects of the Napoleonic wars as far as we were concerned.

JONES: Even now, as Dover bristled with English military might, one weakness remained.

The white cliffs themselves.

If Napoleon did manage to land an army on the coast, how could the English get their troops down to meet him?

It's only about 300 feet from the top of the cliffs to here on the beach, but to get men on horseback down, it's about a mile and a half.

And even to march men down, it's still a mile along narrow winding paths.

Twiss realized there had to be a quicker, better way.

What Twiss designed was an express route from the barracks to the base of the cliff.

It was called the Grand Shaft.

A giant stairwell, 26 feet across and 180 feet deep with three flights of stairs.

It was one of the most brilliant building projects of its day.

God, it actually looks a bit like a sort of futuristic prison.

It's incredible.

MANDY WHAL: They needed a route to move the soldiers very quickly from the barracks site down to the sea front, if Napoleon had invaded.

Originally, they were just chalk paths which would have been really slippery when it was wet.

So what they did, is they constructed ostensibly a well, and put three staircases in it, which meant troops could be moved really quickly from the barracks site above us down to the sea front.

JONES: So by having three staircases you can move troops three times as fast.

WHAL: Three times as fast, yeah.

Can you give us a rough idea of how long it would take to get 1,000 men from top to bottom?

They did do an exercise at one point, and it was to get all the troops from the Grand Shaft barracks site and Dover Castle down to the market square.

And it took 12 and a half minutes.

-12 and a half minutes for 1,000 men?

-Yeah.

JONES: It's almost like water going down a plughole, isn't it?

It is very much like that, yes.

Very, very simple, which is part of the genius.

JONES: Though fully prepared, Dover's defenses were never put to the test.

Blockaded by the Royal Navy, unable to control the English Channel long enough to get his army across, Napoleon was forced to cancel the invasion.

By 1805, the threat had passed, but the fear lingered on.

In the early 19th century, there were real fears that Napoleon would tunnel all the way from France, to here at Dover for an invasion.

And that's not as mad as it sounds because later in the century, in the 1880s, a test tunnel was actually dug more than a mile under the sea before it was canceled on the grounds of national security.

So this is an article from one of the London papers in 1882 discussing the proposed tunnel that was going to be dug under the Channel.

And there's an even interview with a man called Sir Garnet Wolseley.

He thought a tunnel would be "a considerable source of national danger.

"At the dead of night, a small force might be landed at Dover," he said.

"And no one might suspect they're coming, "until they knocked at the door of the fortress."

For the next 130 years, Dover Castle remained unchallenged.

Its tunnels abandoned and all but forgotten.

But the threat of invasion would return.

Once again, it would come from France, but not from the French.

(ADOLF HITLER SPEAKING GERMAN) JONES: For nine centuries, the walls of Dover Castle have adapted to the upheavals of civil war and the threat of foreign invasion.

But at the start of World War II, the protection of this great fortress would extend beyond England, right across the Channel to France.

In May, 1940, the British army was facing almost certain annihilation on the beaches of Dunkirk.

Here, on the coast of northern France, 400,000 Allied troops were now surrounded by a German army twice their size.

And Hitler's forces were closing in for the kill.

Prime Minister Winston Churchill needed a rescue mission, and the man he chose to formulate it was Vice Admiral Bertram Ramsay.

Ramsay's headquarters needed to be as close as possible to Dunkirk.

He chose Dover Castle.

Beneath the white cliffs, the 100-year-old network of tunnels built during the Napoleonic Wars were still a secret to the outside world.

Now, they would be transformed into Ramsay's military command center.

The decision was taken to evacuate Dunkirk as quickly as possible, and the retired Vice Admiral Ramsay was put in charge of an impossible task.

They called it Operation Dynamo, and the first planning meeting took place in these rooms deep below Dover Castle on the 20th of May, 1940.

Ramsay's operation, named after the dynamo room that powered the tunnel system, was planned in just six days.

Immediately, he began to commandeer a fleet of carriers and destroyers for a seemingly impossible mission.

In this labyrinth of tunnels, there were operation rooms for the Army, Navy, and Air Force.

Plans were made here to offer as much cover as possible for the evacuation.

Ramsay and his team were living in these tunnels 24 hours a day.

It must have been incredibly intense and claustrophobic.

MAN: Here's the latest from Dunkirk.

JONES: And even with intelligence reports coming in from the outside, they still had no real way of knowing what was happening to the Allied forces gathering on the beaches of Dunkirk.

Sergeant, inform all gun-sights to stand to.

Almost half a million lives were at risk.

Even at his most optimistic, Ramsay hoped to rescue 30,000 or 40,000.

As well as the German forces, the ships would face the treacherous shallows of the French coast, and an English Channel littered with mines.

(INDISTINCT CONVERSATION) From his bunker deep below Dover Castle, Ramsay gave the order for Operation Dynamo to begin.

And with that, a fleet of 35 destroyers and troop carriers set out for Dunkirk, 47 miles away.

Once there, they would anchor offshore and use landing craft to ferry the soldiers out.

These dunes, in fact the whole beach, were swarming with men under German bombardment, but the inner harbor was out of commission, so the larger ships couldn't approach the beach to rescue the men.

On top of that, the quickest way back to Dover was too dangerous because the sea was mined and there was constant fire from German positions along the coast.

It's almost impossible to imagine the chaos, fear and panic of that first day unless you were there.

97-year-old Vic Viner was among the crew of a landing craft rowing soldiers from the beach to the destroyer HMS Esk, anchored further out to sea.

VINER: The first day, they got 7,000.

JONES: Right.

When we looked round and found we'd been sweating blood, it was... -You'd been sweating blood?

-Oh, yes.

My colleague next to me said, "Vic," he said, "your hands are all covered in blood."

I said, "So are yours."

So we rolled up our sleeves and it was all the way down.

VINER: When all this was happening, over came the Luftwaffe and did their bit.

Quite a few were, um, killed by the bombing, then there was a good number that just walked into the sea going home.

-Just go... -"I'm just going back to England," and off they went.

I can vividly remember two big sergeants, six foot-odd crying, -because they'd had enough.

-Really?

And yet, once you sort of said something to them and made them look up, this is where their, um, discipline came back, you know.

JONES: After three days, around 70,000 men had been rescued, but that still left hundreds of thousands stranded on the beach.

But back in the tunnels at Dover Castle, the second part of Ramsay's plan was about to come into play.

All across the southeast coast and Thames estuary, the Admiral had put out an emergency call for privately owned small boats.

Already, he'd assembled 700 of them at Ramsgate, everything from speedboats and privates yachts to car ferries.

By May 28th, they were headed for Dunkirk.

So what's the idea behind the little ships?

Well, what happened was, when Ramsay called for the Navy to go and evacuate the troops from Dunkirk, they realized that they couldn't get into the beaches and the problem with the larger ships is that they had a depth of 10, 12, 20 feet.

And they couldn't get into the beaches, the troops couldn't get off, so what you needed were the smaller vessels to go into the beach, pick up the troops from the beach and ferry them back to the larger ships.

So these boats were doing lots of little journeys back and forth full of troops being physically taken off the beaches.

That's exactly right.

JONES: And we know something about this particular boat, don't we?

We know how many people were on it?

Yes, unusually, most boats we don't know much about.

The New Britannic, we know that she collected up to 3,000 troops, which is a staggering number of troops, and therefore, an awful lot of young men owe a lot to this fine old vessel.

The larger of the yachts would take as many as 180 or 200 on board and then take them out to the ships, to the larger ships, the destroyers and the merchantmen, which were waiting.

30th of May, um, we got 68,000 away during that day.

JONES: That's incredible.

VINER: And the next day, just under 68,000.

It must have looked like a scene from hell.

That's right.

Oh, yes.

It's very difficult... It's very difficult now to... I try very hard to explain, you know, how I felt.

Um... But it's so peaceful now, it's very difficult.

-Hmm.

-But, um... But in a way, it's peaceful because of what happened -all those years ago.

-That's right, yes.

And I'm just very humbled to know that I was part of it.

JONES: On the 4th June, the last destroyer left Dunkirk harbor, headed here for Dover with the last of the men to be evacuated.

Vice Admiral Bertram Ramsay had hoped to save at most 45,000 lives.

In the end, his Operation Dynamo saved 338,226 people.

CHURCHILL: Sir, we must be very careful not to assign to this deliverance the attributes of a victory.

Wars are not won by evacuations.

But there was a victory inside this deliverance which should be noted.

JONES: In pure military terms, Dunkirk was humiliating, but its success saved the core of a professional army and more importantly than that, what happened between Dunkirk and here at Dover helped create a pride that Dunkirk Spirit and that probably changed the course of the war.

CHURCHILL: We shall defend our island whatever the cost may be.

We shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets.

We shall fight in the hills.

We shall never surrender.

JONES: By the end of 1940, the castle was on the front line of Britain's defenses.

As the Germans dug in along the French coast, Admiral Ramsay knew the secret tunnels beneath Dover Castle were crucial to the war effort.

So he set about expanding them.

Over the old Napoleonic tunnels, a new annex level was carved out with underground living quarters, bathrooms, hospitals and operating theaters.

(ALARM WAILING) From here, radio operators would transmit fake military commands and other disinformation to confuse the Germans.

The work done here would be crucial in the run up to the Normandy landings.

Dover was used as a dummy launching point for D-Day.

So, down in the harbor, there were hundreds of fake landing craft, and thousands of fake orders and communications were sent through this repeater room.

Even when it was pretending, Dover was at the heart of the war.

Yet again, Dover had played a vital role in the defense of the realm.

Thanks to Bertram Ramsay, it had saved not only the British army, but Britain itself.

Bertram Ramsay was killed in a plane crash in January, 1945, so he never saw the end of the Second World War or celebrated the victory he helped to bring about.

But what he did and the part played by Dover Castle in defending Britain is still remembered today.

MAN: Four, three, two, one, zero.

But soon, the world would fall under the shadow of a new kind of war.

And if the worst ever happened, Dover Castle would be the best place to hide.

For almost 900 years, Dover Castle has been Britain's most strategically important fortress.

In all that time, it's stood firm in the face of invasion.

In May 1945, the war in Europe ended with Germany's unconditional surrender to Allied forces.

By August, two atomic bombs ended the war for Japan and ushered in a sinister new age.

Tunnels beneath Dover Castle, once the nerve center of secret military operations, would soon have a new role to play.

One of the main advantages of the tunnels was their secrecy and after the war, this came into play.

The lowest level, Level D, was code named DUMPY and it was to be used as a regional command center in the event of the outbreak of nuclear war.

Now, most of the information about these tunnels is still protected by the Official Secrets Act.

And they don't let the public down here very often.

As the Cold War intensified, the British government began to plan for the possibility of a nuclear attack.

Britain would be divided into 12 designated regional seats of governance.

In 1968, Level D beneath Dover Castle, became one of them.

150 feet below the surface, seven corridors and over 30 rooms were modified to provide shelter for 300 government and military officials in the event of a nuclear strike.

Here, deep inside the white cliffs, Doomsday rehearsals and civil defense training were carried out throughout the '60s and '70s.

If you found yourself down here during the Cold War, then the worst had probably happened.

You'd have been whisked out of your bed in the middle of the night, brought to this bunker to help keep whatever remained of southeast England going.

Fully stocked and equipped with filtration and ventilation systems, conditions were spartan and fairly claustrophobic, but practical in the case of nuclear Armageddon.

The inhabitants of Level D would have been isolated, but still capable of broadcasting to the world outside, if there was one.

This is the BBC Radio studio and they had instructions that in the event of nuclear holocaust they were to play light, upbeat music to keep the people's spirits up.

(UPBEAT MUSIC PLAYING) There was just one problem.

By the 1970s, it was realized that while the chalk surrounding the bunker would have been enough to keep out Napoleon and Hitler, it was also permeable, which meant that any radioactive rainwater from a nuclear disaster above would eventually have seeped through the rock and into the bunker below.

Thankfully, Dover Castle has never had to defend anyone from nuclear attack.

For the last 60 years of peacetime, it has remained a monument to Dover's 2,000-year-old reputation as the gateway to Britain.

In all that time, Dover has contributed more to the defense of Britain than any other castle.

From Roman lighthouse and Saxon fort to the greatest and most formidable castle of the Middle Ages, from royal palace to underground barracks and one of the most ingenious military bases of the 19th century, Dover Castle has extended its defenses well beyond these medieval walls, from the white cliffs to the shores of Normandy.

Today, the Key to England remains a testament to Britain's determination, resilience and bravery.