Tomogashima (Yura Fortress Battery Ruins)

Tomogashima (友ヶ島) is the collective name for a group of four uninhabited islands — Jinoshima (地ノ島), Okinoshima (沖ノ島), Torajima (虎島), and Kamishima (神島) — situated in the Kitan Strait (紀淡海峡) between the city of Wakayama on the Kii Peninsula and Awaji Island. The islands sit at the southern entrance to Osaka Bay, commanding a narrow waterway that has been of strategic importance for centuries.

Following the Meiji Restoration, the new Japanese government identified the Kitan Strait as a critical chokepoint requiring modern coastal defenses. Under the Shusei Kokubō (守勢国防, "Static National Defense") policy of the 1870s-1880s, which emphasized fortified coastal positions to counter threats from the Qing Chinese Beiyang Fleet and the Russian Empire's Pacific Fleet, the government ordered the construction of the Yura Fortress (由良要塞) system beginning in 1889.

The Yura Fortress was designed in three defensive groups to block passage through the Kitan Strait: one group on Awaji Island, one on Tomogashima in the middle of the strait, and one around Miyama on the western shore of the Kii Peninsula. Over a period of 16 years beginning in 1890, the Imperial Japanese Army constructed five artillery batteries on Okinoshima, the largest island in the Tomogashima group:

• 1st Battery (第一砲台): Located on the northern coast, covering the strait approaches
• 2nd Battery (第二砲台): Positioned near the Tomogashima Lighthouse and the 135th Meridian marker
• 3rd Battery (第三砲台): The largest and most powerful battery, built at the highest point of the island with 360-degree firing capability. Featured underground ammunition magazines, shell hoisting mechanisms, and multiple gun emplacements. This is the best-preserved structure on the island
• 4th Battery (第四砲台): Construction of communication roads, reinforced walls, and exhaust systems began in 1891; 12 cm cannons were emplaced in 1896 and 28 cm howitzers installed in 1900
• 5th Battery (第五砲台): Positioned on the southern coast, with a redoubt also constructed on nearby Torajima

Dedicated ammunition depots for the 3rd and 4th Batteries were completed in 1901. At its peak, approximately 600 soldiers were stationed on the island.

In 1888, Tomogashima was designated a military site and removed from civilian maps. Access by the public was strictly prohibited by the Imperial Japanese Army, and during World War II, the island was erased from detailed topographic maps entirely, earning it the designation of a "phantom island" (幻の島). The Tomogashima Lighthouse (友ヶ島灯台), first erected in 1872, continued to operate throughout the military period.

Despite its formidable fortifications, the fortress was never tested in combat. By the time of World War II, warfare had shifted decisively to aerial combat, rendering the anti-ship coastal batteries largely obsolete. The guns of Tomogashima stood silent throughout the war.

After Japan's surrender in August 1945, all military facilities on Tomogashima were abandoned and turned over to the Allied occupation forces. The structures were left to deteriorate. Over the following decades, the subtropical vegetation of the island gradually engulfed the red-brick tunnels, concrete gun emplacements, and stone magazine buildings, creating the haunting ruin aesthetic that draws visitors today.

The Third Battery (第三砲台), with its arched brick corridors, circular gun platforms now filled with rainwater reflecting the sky, and walls draped in moss and ivy, became the island's most iconic site. Its resemblance to the floating castle in Hayao Miyazaki's 1986 animated film "Castle in the Sky" (天空の城ラピュタ) earned Tomogashima the popular nickname "Japan's Real Laputa" or "Japan's Castle in the Sky."

Tomogashima is now part of the Setonaikai (Seto Inland Sea) National Park. The island is accessible only by ferry from Kada Port (加太港) in Wakayama city. The battery ruins, lighthouse, and hiking trails attract visitors year-round. The Third Battery was selected as a Civil Engineering Heritage site (土木学会選奨土木遺産) by the Japan Society of Civil Engineers in 2003. The island also gained renewed attention as the real-life inspiration for the anime "Summer Time Rendering" (サマータイムレンダ, 2022).