Caerphilly is a behemoth of medieval defensive engineering. One of the largest fortresses in Europe, construction was begun in 1268 and briefly interrupted by a Welsh attack in 1270. Its purpose was to secure the Southern Welsh lowlands for its builder, 'Red Gilbert' de Clare. Its 30 acre site is almost without parallel in Britain, and its water defences are unrivalled by any other European fortification.

The castle sits on three artificial islands surrounded by a lake created by damming the Nant Y Gledr stream. The eastern approach is protected by the dam itself, which is in turn protected by an imposing buttressed wall and an outer moat. The Western approach is protected by 'The Horn Work' - a walled island sitting between the attacker and the inner island. This central island plays host to the main castle, which is further defended by an outer wall of its own. The main body of the castle is then protected by four imposing towers - one placed at each corner - with a wide field of fire. The large body of water flanking the castle to the North and South made approach from those directions impossible. When considered hand-in-hand with the ingenious strategic intricacies of the castle's defences, these features make for an imposing and effective military installation that must have struck fear into any attacker who saw it.

Remarkably, the construction of the castle was completed within 40 years - an astounding feat of engineering for the time. Despite the defeat of the Welsh by Edward I in 1283 undermining the strategic importance of the castle, it continued to serve as an administrative centre for the area and saw large-scale action on a number of occasions.

In 1316, Llywelyn Bren attacked the castle with an army of 10,000 men. He failed to breach its walls, but laid waste to the town of Caerphilly.

In 1327, the castle was besieged by Queen Isabella during her campaign to take her husband's throne. 120 defenders were able to hold the castle against an attacking army of thousands. They surrendered gracefully only when offered a free pardon.

By the start of the 15th Century, the Welsh had not posed a serious military threat to the English for over a century. Accordingly, by the 15th Century, Caerphilly - along with many other Welsh castles - had slipped into a state of neglect. The costs of repairing, maintaining and garrisoning fortifications of this size would have been vast. It's not surprising therefore that when Owen Glyndwr attacked in 1403, the defenders quickly surrendered. They were probably little more than a token force occupying a symbolic shell of a fortress. After briefly serving as Owain's headquarters, the castle slips out of military history.

In the years that followed, the castle slowly deteriorated. By the end of the 16th Century, the Lewis family was plundering stone from the castle to improve their home at Van Mansion. The final blow came during the civil war when Parliamentary forces breached the towers, drained the lake and destroyed the castle mills to prevent it being used as a Loyalist stronghold.

The turn in the castle's fortunes came in 1871 when the Marques de Bute began restoration work, re-roofing the main hall. This work continued through the following decades culminating with the re-flooding of the moat in 1958.

Today, it doesn't take much imagination to appreciate what a daunting challenge this castle would have posed to an attacking army. Enough of the fortifications survive to give a clear impression of its impregnability. Added interest is provided by the reconstructed hoardings above one of the castle walls, which provide an insight into an oft-forgotten defensive feature. Also worth seeing are the reconstructed medieval siege engines.