The founding year of Amagasaki Castle is traditionally considered to be 1617, when Toda Ujikané built his castle here, making it the administrative center of the Amagasaki Domain. However, as early as the Sengoku period, a fortress built by the Hosokawa clan already stood on this site. After the fall of Itami Castle in 1579, Araki Murashige—formerly a vassal of Oda Nobunaga who had rebelled against him—fled to this earlier castle.
Mōri Terumoto (1553–1625) was the grandson and rightful heir of the renowned Mōri Motonari. When Terumoto became the head of the Mōri clan in 1571, he inherited vast territories covering a large part of the San’in and San’yō regions in western Honshū. In addition, the Mōri clan possessed the largest and most technologically advanced naval fleet of its time.
After the defeat of Toyotomi Hideyori’s supporters in the Osaka Campaigns of 1614–1615, many clans in Japan still remained not fully loyal to the Tokugawa shogunate, especially in the western Chūgoku region. Mizuno Katsunari (1564–1651), a cousin of Tokugawa Ieyasu, became the first of the Tokugawa house’s close retainers, the so-called fudai daimyō, to be relocated to this strategically important area.
The founder of the Chiba clan is considered to be Chiba Tsunesige (1083–1180), who in 1126 moved his residence to the Inohana area and built a strongly fortified stronghold there. Although Tsunesige himself came from the Taira clan, the Chiba clan later supported Minamoto no Yoritomo, the future founder of the first shogunate.
Oda Nobunaga, as part of his “final solution” to the conflict with the Saitō clan, conceived the construction of a fort in the Sunomata area, which was intended to serve as a forward base for an attack on Inabayama Castle (later renamed Gifu). Sunomata was a swampy area located between Ogaki Castle, a stronghold of the Oda clan, and Inabayama, the main fortress of the Saitō clan. Earlier attempts to build fortifications in this area, carried out by Oda generals Sakuma Nobumori and Shibata Katsuie, had all ended in failure. After that, the task was entrusted to the young vassal Kinoshita Tokichirō, who later became known as Toyotomi Hideyoshi.
The exact time when structures first appeared on the site of the present-day castle is unknown; however, it is generally believed that the first fortified buildings were constructed here during the Muromachi period, when these lands were controlled by the Shibata clan.
