Castles went through various stages of development from simple ‘motte and bailey’ constructions to the vast stone edifices constructed in Wales during the thirteenth century. The concentric plan in the great Welsh castles involved the principle of successive lines of defence so that each ‘ward’ or section of the castle was placed wholly within another. The outermost defence was the curtain wall, surrounding the whole castle, studded with protective towers. The entrance to the castle was protected by moat, ditch, drawbridge and portcullis. The portcullis was operated within a massive gatehouse, which was flanked by great stone towers and could house troops.