In the fifth month of 1615 (Keicho 20), Tokugawa Ieyasu forced Toyotomi Hideyori to suicide at Osaka Castle, eliminating the last rival to the Tokugawa order. In the immediate aftermath, Ieyasu set the policy of encircling Osaka with Tokugawa-loyal castles — directing the construction or reinforcement of fortresses at Amagasaki, Akashi, Takatsuki, and Yodo to deter western daimyo and quash any Toyotomi revival. Ieyasu died in April 1616 at age 75, but his successor Hidetada swiftly carried out the policy. Amagasaki Castle, ordered in 1617, was among the first results — a western anchor of the ring Ieyasu had designed. The castle embodies Ieyasu's final strategic vision: that once the Toyotomi were gone, Osaka's flanks must never again be left unguarded.

The primary reason Amagasaki Castle was built is the Tokugawa's fear of Toyotomi Hideyoshi's legacy. Hideyoshi died in 1598, leaving the five-year-old Hideyori as his heir and Japan as his bequest. Tokugawa Ieyasu neutralized this at Sekigahara (1600) and Osaka (1614-15), destroying the Toyotomi clan. Amagasaki Castle was ordered built the very next year. Positioned approximately 12km west of Osaka Castle — Hideyoshi's great fortress — it was designed to watch and contain the city that had been Hideyoshi's power base. Amagasaki Castle's very existence was a declaration of the Toyotomi order's end and the Tokugawa era's absolute beginning.