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.

Below: southern view of the castle showing Caerphilly's postern gate (center) and the leaning south-east tower (right).

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.

Below: general view of Caerphilly's inner ward showing the inner west gatehouse (left) and the north-west tower (right).

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 (below left). 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 (below right) 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.

Below (2): view of the southern dam at Caerphilly and view of the three great towers along the northern dam

Below (2): the outer east gatehouse at Caerphilly, the main entrance to the castle and the south gatehouse leading to the town.

Next is the outer ward entered via a bridge leading to its east gatehouse with twin D-shaped towers, only the restored outer walls of which remain. Its opposite number on the west (right) is slightly better preserved. There, the stone piers supporting the drawbridge leading to the hornwork can be seen, as can chimneys and fireplaces on two floors, indicating comfortable living quarters. The false machicolation, poorer masonry and chimneys may indicate that the gatehouse was rebuilt at a later date. In the south-east corner is the base of a large rectangular building, possibly a granary. The south side is entirely blocked by a two-storey D-shaped kitchen tower, stores and servants’ quarters. Below is a water-gate to the lake.

Below (2): view of the outer west gatehouse from the hornwork and view of the restored north-west tower at Caerphilly

The imposing east gatehouse of the inner ward is the climax of any visit to the castle. It consists of twin D-shaped towers, a central passage with portcullises at both ends, and circular stair turrets on the inner corners. Like much of the inner ward it was ruinous by the 19th century, but was meticulously restored by the 4th marquis of Bute. The handsome and comfortable apartment on the second floor was probably that of the keeper, or constable of the castle. On the opposite side

Below: the inner east gatehouse at Caerphilly.

On the south side of the inner ward are the great hall and state apartments. The large ground-floor hall, which was evidently a sumptuous building, was remodelled by Hugh le Despenser the Younger in 1322-6 and was restored by the 3rd marquis of Bute in the late 19th century. Originally the timber roof was lower, carried on the four carved corbels still in place in the south wall. Hugh le Despenser brought in the best craftsmen, who raised the roof and gave the four windows a decorated ogee shape, rich mouldings, and glass. The door was treated in the same way, and the whole building was faced with ashlar. The two doors at the east end led to a buttery and cellar, possibly with a small chapel over them. To the west were the state apartments, well-appointed rooms with fireplaces and a large traceried window on the first floor.

Below (2): exterior view of the great hall from the inner ward (left) and interior view of Caerphilly's great hall (right)

Below (2): restored parapet on the north side of the inner ward Below: working siege engines on display at Caerphilly Castle.