The design of the inner bailey, with its wall studded with semi-circular projections, is unparalleled. This innovation had two advantages: firstly, the rounded wall absorbed the damage from siege engines much better as it did not provide a perfect angle to aim at; secondly, the arrowslits in the curved wall allowed arrows to be fired at all angles. The inner bailey, which contained the main residential buildings, used the principles of concentric defence. This and the unusual design of the inner bailey's curtain wall meant that castle was advanced for its age, since it was built before concentric fortification was fully developed in Crusader castles such as Krak des Chevaliers. Concentric castles were widely copied across Europe; for instance when Edward I of England—who had himself been on Crusade—built castles in Wales in the late 13th century, four of the eight he founded were concentric.