"Medieval concentric castle walls were one feature that distinguished them from other forms of medieval castle. The outer wall was called the “curtain wall” and contained projecting towers which served as the first line of defence in case of an attack. The thickness of the curtain wall could be anywhere between 6 to 20 inches. The inner walls of the medieval concentric castle were higher which allowed the defenders at those walls to observe the activity on the outer wall and plan the defence accordingly."