Caerphilly Castle () is a medieval fortification in Caerphilly in South Wales. The castle was constructed by Gilbert de Clare in the 13th century as part of his campaign to conquer Glamorgan, and saw extensive fighting between Gilbert and his descendants and the native Welsh rulers. Surrounded by extensive artificial lakes – considered by historian Allen Brown to be "the most elaborate water defences in all Britain" – it occupies around and is the second largest castle in Britain. It is famous for having introduced concentric castle defences to Britain and for its large gatehouses.