The central island held Caerphilly's inner defences, a roughly square design with a walled inner and middle ward, the inner ward protected by four turrets on each of the corners. The walls of the inner ward overlooked those of the middle ward, producing a concentric defence of two enclosed rings of walls; in the medieval period, the walls of the middle ward would have been much higher than today, forming a more substantial defence. Caerphilly was the first concentric castle in Britain, pre-dating Edward I's famous programme of concentric castles by a few years. The design influenced the design of Edward's later castles in North Wales, and historian Norman Pounds considers it "a turning point in the history of the castle in Britain". Probable subsidence has caused the south-east tower in the Inner Ward to lean outwards at an angle of 10 degrees.