The keeps were ideal for the time in which they were built, but by the middle of the 13th century needs had changed. A base that could be used for offensive operations rather than as a purely defensive stronghold was needed. So the keep was discarded in favour of a concentric design. Concentric castles have no central strong point like a keep. Instead they rely on rings of walls, one inside the other, with towers along the length of the walls.
Most Edwardian castles have three concentric rings of walls and towers.
The central space was kept as an open courtyard around which were clustered separate domestic buildings. The outer wall was ringed by a moat with access over a draw bridge through a separate gatehouse or barbican. Several Norman keeps were converted into concentric castles. The central keep was retained for accommodation. Palaces and Manors.