Ō-yoroi first started to appear in the 10th century during the middle and late Heian period, and came into widespread use in the Genpei War around the 12th century when the call for armor was at its peak. Significant aspects of this armor were designed for cavalry archers. The box shaped ō-yoroi was heavy and did not allow as much movement or flexibility as its counterpart the dō-maru, so the armor fell out of favor in the fifteenth century when samurai shifted to mostly infantry tactics.(Ogawa, 1989).
