Stone walls were used at several important parts of a citadel even in the medieval period, but stone walls reinforced by stacking stones on the surface of earthen walls developed as the need arose to build heavy turrets near the citadel in the Azuchi-momoyama period.
石垣は中世においても城郭の要に一部用いられることはあったが、安土桃山時代になると、重い櫓を郭の際に建てる必要から、土塁の表面に石材を積んで強化した石垣が発達した。