Caerphilly Castle is one of the great medieval castles of western Europe. Several factors give it this pre-eminence - its immense size (1.2h), making it the largest in Britain after Windsor, its large-scale use of water for defence and the fact that it is the first truly concentric castle in Britain. Of the time of its building in the late 13th century, it was a revolutionary masterpiece of military planning

One of Henry III's most powerful and ambitious barons, Gilbert de Clare, lord of Glamorgan, built this castle. His purpose was to secure the area and prevent lowland south Wales from falling into the hands of the Welsh leader Llywelyn the Last, who controlled most of mid and north Wales.

De Clare built other castles on the northern fringes of his territory for the same purpose, such as Castell Coch. He had seized the upland district of Senghenydd, in which Caerphilly lies, from the Welsh in 1266 to act as a buffer against Llywelyn's southward ambitions. Llywelyn realised the threat and tried but failed to prevent the castle from being built; it was begun on 11 April 1268, was attacked by Llywelyn in 1270, and was begun again in 1271. This time it was completed without hindrance.

Its message was not lost on Llywelyn, who retreated northwards. Apart from the remodelling of the great hall and other domestic works in 1322-6 for Hugh le Despenser, no more alterations were carried out, making it a very pure example of late 13th-century military architecture.

Caerphilly is unusual in being a late castle built on a virgin site. This allowed a unity of conception rare in medieval castles. It is a double-skinned parallelogram surrounded by large-scale water defences. The concentric arrangement was more flexible than earlier plans. It gave rapid access to any part of the castle by mural passages and wall-walks, towers and gatehouses could be independently held, attackers could be well covered and there was no possibility of mounting siege engines against the inner walls.

The castle's cellular structure and strength is indicated by the presence of numerous portcullises. The outer skin or ward is formed by a low battlemented curtain wall with large semi-circular projections in the corners and gatehouses in the middle of the east and west sides. Only a narrow strip separates this from the much stronger inner ward which has high curtain walls, circular corner towers and two large strong gatehouses corresponding with the outer ones.

The great east gatehouse is the highest part of the castle and was its nucleus. As will be seen, it could be separately defended if necessary. The south and north lakes around the castle formed an almost insuperable barrier to attackers. The dams themselves are a major achievement of medieval engineering. The southern, earliest one is a massive earth platform revetted in stone and strengthened on its lower side by eight great buttresses.

To the right of the entrance to the castle is the northern dam, a narrower platform with a high outer wall with three great towers which are now unfortunately suffering from subsidence on the marshy ground. At its end is a strong postern gate and drawbridge. Outside the dam is a moat fed by sluices in the southern dam.

The castle's active history was an extremely short one. By 1283 Edward I had removed the threat of Welsh independence and the need for Caerphilly had gone. Minor Welsh attacks in 1294-5 and 1316 failed to make any impact. The last action that Caerphilly saw was in the war between Edward II and his queen, Isabella. Intent on destroying the power of her husband and his favourite Hugh le Despenser, Isabella besieged the castle from December 1326 to March 1327.

But by this time Edward had fled and Hugh had been hanged. Thereafter the castle declined and fell into ruin. In the late 16th century Thomas Lewis of The Van, just outside Caerphilly, was granted permission to use its stone to build his new house, thus accelerating its dilapidation. In the Civil War it was unusable and an earthwork redoubt was built instead to the north-west, the remains of which are still visible in the trees beyond the north lake.

By the 18th century the lakes were dry and houses had been built against the foot of the south dam. That the castle rose again from its sorry state is due to the visionary clearance and restoration work undertaken by the Bute family and the imaginative reflooding of the lakes by the state in the 1950s.

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