"Since a concentric castle consisted of outer and inner walls, this kind of medieval castle provided a solid defence. The inner wall could be defended from the outer wall and once the outer wall was penetrated by the enemy, the inner wall could still be used to protect the actual castle. The rounded or polygonal shaped keeps were particularly important because they eliminated the risk of mining underneath the corners which was a huge advantage. Another important factor for defence was the presence of multiple walls, with the inner most wall being the highest. This allowed the defence of the medieval castle to be established on multiple levels at the same time, making it virtually impenetrable."