When the Mori clan began invading Izumo in 1562, Mitoyajjo Castle served as the base for war supplies for the Mori force, as the castle was a strategic point connecting the place in the shade of the mountain and the south side of the mountain.
永禄5年（1562年）に毛利氏が出雲への侵攻を開始すると、三刀屋城は山陰と山陽を結ぶ要地であったため、毛利軍の兵站の拠点となった。