Kurushima Castle was a sea castle of the Kurushima Murakami clan, encompassing the entire island of Kurushima, a small island located at the southern end of the Geiyo Islands, right next to Shikoku. It served as a strategic castle to protect the narrow Kurushima Strait between the island of Ōshima and Shikoku. The Kurushima Murakami clan originated from the navy of the Kono clan, the governor of Iyo Province. Later, they served Hideyoshi and became daimyo with a domain of 14,000 koku in the Kazahaya district of Iyo Province. After the Battle of Sekigahara, they were transferred to the Bungo-Mori domain and adopted the name Kurushima.