Jōgashima Artillery Battery Ruins (Tokyo Bay Fortress)

Jōgashima (城ヶ島, "Castle Island") is a natural island located at the southernmost tip of the Miura Peninsula in Kanagawa Prefecture, connected to the mainland by the Jōgashima Ōhashi bridge. Strategically positioned at the western entrance to Tokyo Bay, the island has served as a military observation and defense point since the Edo period.

The island's military significance dates back to 1808 (Bunka 5), when the Uraga Magistrate Iwamoto Masanori established a battery for monitoring foreign ships. Following the Meiji Restoration, the Imperial Japanese Army incorporated Jōgashima into the Tokyo Bay Fortress (東京湾要塞) — a comprehensive coastal defense network comprising batteries and sea forts spanning both sides of the bay entrance, from the Miura Peninsula in Kanagawa to the Bōsō Peninsula in Chiba.

The modern battery was begun on October 12, 1924 and reported complete on April 2, 1927, as part of the post-Great Kantō Earthquake reconstruction of the Tokyo Bay Fortress. The battery was armed with two twin-mount 25.4 cm (10-inch) 45-caliber gun turrets — secondary-battery wing turrets reused intact from the Satsuma-class battleship Aki (安芸), which had been struck off under the Washington Naval Treaty of 1922. These guns had a maximum range of approximately 24,600 meters at 30 degrees elevation. The gun positions were elaborately camouflaged with painted surfaces and false structures, including dummy houses, to conceal them from aerial observation. However, the Second Battery experienced progressive tilting problems shortly after completion, with the issue reported to the Army Minister in December 1927.

As part of the Tokyo Bay Fortress, Jōgashima Battery worked in conjunction with other installations including Sarushima Battery, Chiyogasaki Battery, the Kannon-zaki batteries, and the three Kaiho sea forts to create an overlapping field of fire covering the approaches to Tokyo Bay. The fortress system was designed to protect the capital Tokyo, Yokosuka Naval Base, and other strategic assets along the bay from naval attack.

Throughout World War II, the battery remained operational as part of the capital's defense network, though the Tokyo Bay Fortress never engaged enemy naval forces in combat. Following Japan's surrender in August 1945, all military installations were decommissioned and dismantled under Allied occupation directives. In 1958, the island was converted into Jōgashima Park (城ヶ島公園). While surface-level military structures were largely removed, significant underground facilities survive beneath the park's first parking area, including three ammunition magazines and a connecting passage of approximately 50 meters.