In the heart of South Wales, there’s a castle that most visitors to Britain walk right past. It doesn’t have the royal glamour of Windsor or the famous name of Edinburgh. What it does have is a leaning tower more tilted than the one in Pisa, a moat so enormous it was once described as an inland sea, and a history fierce enough to make your jaw drop. Welcome to Caerphilly Castle — arguably Britain’s most underrated fortress.
A revolutionary design that changed medieval warfare
When Gilbert de Clare broke ground at Caerphilly in 1268, he wasn’t just building a castle. He was inventing a new kind of warfare.
De Clare, the powerful Norman Lord of Glamorgan, needed to control the surrounding Welsh lands and hold them against Llywelyn ap Gruffudd, the last native Prince of Wales. His solution was radical: a concentric fortress surrounded by an artificial water system unlike anything previously built in Britain.
Rather than a single ring of defensive walls, Caerphilly was ringed by multiple layers — inner walls, outer walls, water gates, and island platforms. Every approach required crossing water, which neutralised siege engines. Every gate was covered by another wall. The sheer engineering ambition of the place is still remarkable 750 years later.
Today, Caerphilly is the second largest castle in Britain after Windsor. Covering roughly 30 acres including its water defences, it’s also one of the best-preserved concentric castles in the world. To understand how unusual this made Wales in the medieval period, Wales’s extraordinary castle density tells the full story.
The tower that shouldn’t still be standing
Walk into the outer ward and look south. The south-east tower leans at roughly 10 degrees off vertical. The Leaning Tower of Pisa, for context, tilts at around 4 degrees. Caerphilly’s tower is more than twice as dramatic — and unlike Pisa, the lean wasn’t accidental.
During the English Civil War in the 1640s, Parliamentarian forces tried to demolish Caerphilly to prevent Royalists using it as a stronghold. They undermined the south-east tower’s foundations. The plan half-worked. The tower cracked, lurched dramatically — and stopped. It has been frozen at that impossible angle for nearly 400 years.
There’s something almost defiant about it. The tower was supposed to fall and never did.
A moat that became a lake
Caerphilly’s water defences aren’t a simple ditch. They’re an engineered network of lakes created by damming local streams across a low-lying valley. At full extent, the lakes covered around 30 acres, turning the castle into what medieval chroniclers called an island fortress.
The outer lake was designed to stop siege engines — no wheeled machine could be dragged through deep water. The inner island gave the garrison a second perimeter to defend. It was a system designed to exhaust and frustrate any besieging force before they reached the first wall.
Today the lakes are still there, maintained by Cadw, the Welsh Government’s historic environment service. On still mornings, the towers reflect perfectly in the water, making the castle look twice as tall. It’s one of the most photogenic sights in Wales — and almost nobody outside Britain knows it exists.
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The history that forged it
Caerphilly saw serious action across the centuries. Welsh rebel leader Owain Glyndwr attacked the outer works during his famous revolt in the early 15th century, and his forces did genuine damage that was never fully repaired — which explains why parts of the outer ward still look dramatically ruined rather than simply old.
Edward II, one of England’s most troubled monarchs, sheltered here in 1326 while his wife Queen Isabella and her lover Roger Mortimer moved against him. The great hall would have been one of the last places he felt any security before his eventual capture. Life inside a medieval castle was rarely comfortable, but for Edward II, Caerphilly was briefly a refuge.
The castle later fell into slow decline. By the Victorian era it was overgrown and crumbling. The third Marquess of Bute — the eccentric coal magnate who also restored Cardiff Castle — began clearing and stabilising the site in the 1870s. His family eventually donated it to the state.
Planning your visit
Caerphilly Castle is easy to reach from Cardiff — about 20 minutes by train on the Rhymney Valley line, or a short drive north. The town of Caerphilly is pleasant, with good cafes and the famous local cheese market nearby.
The site is open year-round with seasonal hours. Entry is affordable by British heritage standards. Inside, there are life-size replica siege weapons positioned across the grounds — trebuchets and mangonels that make the scale of medieval siege warfare viscerally real. For a guided tour of Caerphilly Castle, Viator has several options worth exploring.
If you’re touring Welsh castles more broadly, Beaumaris Castle on Anglesey is another marvel of medieval military engineering — architecturally perfect but never finished.
Frequently asked questions
Is Caerphilly Castle worth visiting?
Absolutely. It’s one of the finest medieval castles in Britain and one of the most underrated in all of Europe. The combination of enormous water defences, a dramatically leaning tower, and genuine medieval atmosphere makes it exceptional. It’s far less crowded than comparable castles in England, which makes exploring it all the more enjoyable.
Why does Caerphilly Castle lean?
The south-east tower leans roughly 10 degrees because of deliberate but incomplete demolition during the English Civil War in the 1640s. Parliamentarian forces undermined the tower’s foundations to prevent Royalists using the castle as a stronghold. The demolition was never finished — and the tower has stood at that dramatic angle ever since, more tilted than the Leaning Tower of Pisa.
How does Caerphilly Castle compare to Windsor?
Windsor is the largest castle in Britain and still in active royal use. Caerphilly is the second largest. What makes Caerphilly distinct is its extraordinary water defence system — Windsor has no equivalent. Caerphilly’s concentric design, vast artificial lakes, and the drama of the leaning tower give it a character that’s entirely its own.
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Caerphilly Castle has been hiding in plain sight for 750 years. Locals know it. Historians revere it. The rest of the world is still catching up. Go before everyone else does.