in mid-Glamorgan, a baronial castle begun in 1271 by Gilbert de Clare, earl of Gloucester
, is ‘one of the most remarkable defensive complexes ever completed by an individual [private] patron in the Middle Ages
’. This huge castle is an early example of the principle of concentric fortification and heavily defended gatehouse. It is notable for its extensive water defences, a major engineering feat, involving the construction of a fortified dam to hold the waters in place. This aspect of the castle's defence seems to have been influenced by Gilbert's participation in the siege of Kenilworth
in 1266, in which the mere had played an important part in the successful defence of the castle. Caerphilly's scale was possible because Gilbert de Clare was very wealthy. Also as lord of Glamorgan he was subject to the law of the march, which meant that he could build a castle without licence from the king. Further he felt threatened by the possibility of Welsh rebellion and unsure of the king's friendship, as after Simon de Montfort's
rebellion he had promoted the cause of the disinherited, supporters of Simon. The result of particular circumstances, it played no significant part in subsequent Welsh history and on Leland's
visit 1535–45 was in a ruinous condition.