When the castle changed from a medieval period temporary military base to a permanent base of rulership, a defense line was often established outside the functional component (uchiguruwa (the central portion of a castle compound)) of the conventional castle to defend the castle town and vassals.
城が中世の臨時的な軍事基地から恒久的な統治拠点になると、城下町や家臣団防備の目的で従来の城の機能的構成部分（内郭）から、さらにもう一重外側に防御線が設けられることがあった。