Minakuchi Okayama-jo Castle was built in 1585 by Nakamura Kazuuji on Mt. Kojo-san in Minakuchi, Koka City, under the orders of Toyotomi Hideyoshi.
During the Warring States period, the region of Koka was ruled by Koka warriors of equal power who utilized a system of self-government. However, the construction of Minakuchi Okayama-jo Castle marked a major turning point in the history of the region, as the system changed into one that was being ruled by a unified government.
The castles built by Koka warriors in the Warring States period were made of dirt. However, Minakuchi Okayama-jo Castle displayed new features of castles that did not exist in Koka before, built of stone walls and tiled roofs coming from materials gathered from surrounding temples.
The appearance of Minakuchi-Okayama-jo Castle shocked the people of Koka at the time. The castle visually signified the arrival of a new era of rule by a unified government.