Rising from a minor lord of Aki, Mori Motonari made Gassan Toda Castle his objective in a long struggle with the Amago, lords of the San'in. After a crushing defeat in the Tenbun siege (1542–43), his renewed siege from 1565 avoided direct assault and instead cut off supplies in a thorough starvation blockade. In 1566 he forced Amago Yoshihisa to surrender, taking the 'impregnable' Toda Castle and securing hegemony over the Chugoku region.

In 1486 Amago Tsunehisa retook Gassan Toda Castle — from which he had once been expelled — by surprise attack, and through gekokujo (overthrow from below) raised the Amago into the foremost Sengoku house of the San'in. A master of stratagem, Tsunehisa was called the 'sage of scheming,' and in his day the Amago commanded eight provinces at their height. Gassan Toda Castle was the symbol of that achievement.