Concentric castles were designed so that an inner wall was built inside an outer wall, two or more walls were built concentrically around the keep, and inner walls became higher so that the inner defense was effective even if the outer wall was destroyed.
コンセントリック(集中)型と呼ばれる城は、外壁の内側にさらに内壁を加え、天守から同心円状に2重以上の壁をはりめぐらせ、内側に行く程、壁を高くして、外壁を破られても内側の防御が有利になるよう工夫されている。