Amazaki Castle, Maritime fortress in Imabari, Japan

Amazaki Castle is a Japanese island castle (umijiro) on Kojima islet at the eastern tip of Omishima in the Seto Inland Sea, now part of Imabari, Ehime Prefecture. Its stone walls were built in tiers around the islet; at high tide the lower courses are submerged, forming what is called an underwater layout (kaichū nawabari), and only the foundation portions of many walls remain after stones were carried off following the castle's abandonment.

Amazaki Castle began in the Kamakura period as a fort built to police the ama, the seafarers of the Seto Inland Sea, on an island whose old name Amanosaki combines ama (seafarer) and saki (guard). After the 13th-century Mongol invasions its role shifted to controlling maritime traffic, and from the Nanboku-chō into the Sengoku period it served the Murakami navy as a base for stopping ships from the east, inspecting cargo and collecting tolls. Toyotomi Hideyoshi's rise brought it under Tōdō Takatora, whose cousin Yoshikatsu held it until the castle was abandoned when Takatora was transferred in 1608.

The site was the base of the Murakami clan, a sea power that controlled the straits of the Seto Inland Sea from island fortifications. Standing on the remains today, you can see exactly how the position allowed a clear view over the surrounding waters.

The island of Kojima lies about 160 metres off the eastern tip of Omishima and becomes joined to it by a land path only at low tide, a phenomenon called "umiware" that occurs just a few times a year. Time a visit to the tide tables, since at high water the route and the lower stone walls are submerged. Much of the masonry was carried off after the castle was abandoned, so mainly foundation courses remain.

The interior of the fortress was divided by heavy stone walls into separate sections, each sealed off from the others. An attacker who broke into one section still had to fight through every remaining section one by one.