Okinoshima Battery No. 3: Inside Tomogashima's Red Brick Fortress

A Meiji fortress at the gateway to Osaka Bay

Completed in 1892, Battery No. 3 on Okinoshima once helped guard the southern entrance to Osaka Bay. Today, its red brick arches, underground passages and empty gun pits make it one of the most distinctive historic walks in Kansai. Vines hang across walls built for military defense, while sunlight reaches into corridors that once served soldiers and their ammunition stores.

Okinoshima is the largest of the Tomogashima Islands off Wakayama. Battery No. 3, the largest of its five surviving batteries, dominates the western side of the island alongside four other emplacements. Its galleries and vaulted rooms preserve the shape of a coastal stronghold that served the Imperial Japanese Army from the late nineteenth century through the Second World War.

Visitors can look into ammunition magazines, stand inside former gun positions and enter the soldiers quarters known locally as empeibu. Thick masonry and earth embankments sheltered crews, powder rooms and storage spaces. Above the complex, the Takanosu Mountain observatory looks across the Kitan Strait toward Awaji Island.

The attraction now rests partly on that contrast between military architecture and encroaching vegetation. Red brick and deep green foliage have prompted comparisons with Studio Ghibli's Castle in the Sky. Battery No. 3 has also appeared in films and television dramas, drawing photographers, animation fans and a new generation of travelers to Tomogashima.

A visit combines a ferry crossing, a forest walk and an encounter with structures more than a century old. The ruins remain compelling, but access depends on weather, ferry schedules and the condition of individual paths and buildings.

Why the Kitan Strait mattered

Tomogashima stands in the narrow Kitan Strait between Wakayama and Awaji Island. The channel forms a natural funnel at the southern entrance to Osaka Bay. Ships approaching from the Pacific or the Seto Inland Sea had to pass through waters overlooked by Okinoshima.

Osaka and Kobe were the industrial and commercial heart of western Japan in the late nineteenth century. After the Meiji Restoration in 1868, Japan rapidly modernized its armed forces and coastal defenses. Protecting the maritime approach to those cities became a national priority.

The Meiji government developed a network often called the Yura Fortresses. Positions on Tomogashima worked with forts on nearby shores, creating overlapping fields of fire. In simple terms, guns at different locations could cover the same stretch of water, making the passage harder for hostile ships to use.

Battery No. 3 was Okinoshima's flagship position. Site guides and historical accounts put the reach of its coastal artillery at roughly 7.8 kilometers, enough to control ships approaching the strait. Its importance came from the combination of that reach and a location overlooking a narrow maritime gate.

Brick galleries, vaulted ammunition magazines and earth embankments served different protective purposes. Brick resisted weather and shock. Earth covering reduced damage from flying fragments and helped conceal the buildings against the hillside. Tunnels linked storage areas and open gun mounts behind thick walls, creating several layers of protection for soldiers and ammunition.

From military exclusion to public landmark

Tomogashima was strictly off limits during its military service. Local histories describe restrictions so extensive that the islands disappeared from civilian maps. The landscape now explored by day visitors was once a controlled military zone.

The principal dates trace its transition from a defended sea passage to a place of public exploration:

• 1868: The Meiji Restoration preceded Japan's rapid modernization of its armed forces and coastal defenses.
• 1872: The lighthouse on Okinoshima's western tip was built.
• 1892: Battery No. 3 was completed.
• Late nineteenth century through the Second World War: The island served the Imperial Japanese Army.
• After the war: The retired military islands opened in stages and became part of Setonaikai National Park.

Vegetation spread as military use ended. Roots pushed into joints between bricks, and ferns covered former gun platforms. The resulting mixture of architecture and forest now attracts artists, filmmakers and location scouts.

Animation fans compare the ruins with the floating city in Castle in the Sky, while cosplayers use the shadowed corridors for photographs. Travel coverage frequently features the underground vaults and the deeper red tones that afternoon light brings to the brickwork.

Social media posts helped revive day trips from Osaka and Wakayama City. On busy weekends, families and students follow the island paths, connecting the remains of gun positions with the rooms where soldiers stored powder and shells. The same spaces that provide cinematic scenery also make the workings of a nineteenth century coastal defense easier to understand.

Inside the galleries and gun positions

The approach to Battery No. 3 passes through evergreen forest. Summer brings the sound of cicadas and humidity that clings to the stone. Inside the complex, the light fades, interrupted by openings above and shafts of daylight from doorways.

Water sometimes collects on magazine floors, producing reflections that attract photographers. Lamp recesses remain visible in the walls. Arched doorways provided space for moving shells, and the curvature of some long galleries hides the exit from view.

Stairs lead to former gun positions, while passages connect rooms used by crews during wartime. Battery No. 3 retains well preserved brickwork and underground spaces that allow careful exploration in accessible areas. A flashlight is useful because daylight does not reach every corridor.

The neighboring Second Battery presents a different situation. It has suffered heavy damage, and some sections are unsafe to enter. A safety sign at that site gives visitors a direct warning:

Restricted Area. These remains of an artillery battery continue to erode and therefore are very dangerous. Please do not enter this area.

Masonry can fail even when it appears solid. A fall inside a tunnel can also leave a visitor far from quick assistance. Marked paths and barriers protect both people and the surviving structures, so damaged sites should be viewed from outside restricted areas.

Battery No. 3's thicker walls and clear routes make its accessible spaces a better place to spend time. Even there, visitors should avoid touching delicate brickwork and follow posted restrictions rather than treating every doorway as an invitation to enter.

Reaching the island from Kada

Ferries leave Kada Port, northwest of central Wakayama City, for Nonaura port on Tomogashima. The crossing takes about 20 minutes. Travelers coming from Osaka can use the Nankai Line to reach Kada Station, then walk approximately 15 to 20 minutes to the pier.

Adult return fares are often 2,200 yen, with child fares around 1,100 yen. Several departures usually operate each day during the main season, while winter service is reduced. Wednesdays often have no service outside holidays.

Schedules change with the season and weather. The operator publishes official departure information in the Tomogashima Ferry timetable. Checking it before traveling is essential, particularly when planning a connection back toward Osaka.

Reservations for ferry tickets are often not accepted. Visitors should check the last return departure and reach the pier early. Weather can disrupt crossings, so the time available on the island depends on the day's operating conditions.

The ferry is also a practical limit on a day's sightseeing. Time spent photographing the tunnels or resting at an observatory needs to fit around the return sailing, rather than simply the distance left to walk.

Walking the circuit and preparing for limited services

A loop of about 3.3 kilometers connects major sights, including the lighthouse, several batteries and the Takanosu observatory. Most visitors take two to three hours to walk it, allowing extra time for Battery No. 3's underground passages.

The paths have gentle gradients, but surfaces can become slippery after rain. Toilets are available near the pier, and a small cafe operates at times. Other services are limited, so visitors need to bring what they expect to use during the walk.

• Wear sturdy shoes rather than heels or sandals.
• Carry water, snacks and a flashlight for dark passages.
• Check the final ferry departure and arrive at the pier early.
• Expect patchy mobile reception in wooded areas.
• Follow closures after storms, including restrictions on some eastern paths.

Typhoons and heavy rain can close the eastern side of Okinoshima while authorities assess damage. Those restrictions affect which parts of the island can be explored, even when the main ferry route is operating.

At low tide, a route can appear across the shallows toward Torajima and its smaller battery remains. The crossing can be hazardous and is sometimes closed. Visitors need to check local signs, tides and conditions before attempting it, and follow any restriction in force.

Camping comes with strict conditions

Camping is permitted only at the Minami Taramizu campsite, reached by a short forest walk from the pier. Reservations must be made online in advance. Facilities are basic, with no electricity or drinking water.

Campers must bring all equipment because rental gear is unavailable on the island. They also need to carry their rubbish away. Fires, fireworks and beach bonfires are prohibited, and quiet hours apply at night.

• Use only the reserved campsite plot.
• Groups cannot reserve multiple plots.
• Bring all necessary equipment and drinking water.
• Turn lights off after 10 p.m. and avoid parties.
• Take all rubbish back off the island.

Camping may be denied when weather makes a ferry suspension likely. An overnight stay therefore depends on both a campsite reservation and suitable transport conditions.

Wildlife, storms and fragile ruins

Warm weather brings active snakes, including the venomous mamushi pit viper. Deer and squirrels also live on the island. Peacocks were introduced years ago in an attempt to reduce the snake population.

Visitors should give wildlife space and remain on marked trails. Forest paths, old masonry and changing weather require attention throughout the walk, particularly after rainfall or when entering dimly lit spaces.

Currents sometimes carry rubbish onto the coastline. Volunteers and local officials periodically remove debris, but everyone visiting the island should take away everything they bring. Keeping paths and shores clean is part of protecting the national park setting.

The vegetation that gives the ruins their distinctive appearance also grows into aging structures. Roots splitting brick joints are part of the visible transformation from fortress to forest. Respecting barriers and avoiding fragile surfaces helps preserve the surviving architecture while reducing the risk of injury.

A lighthouse, wetland and views toward Awaji

Okinoshima's western tip holds a white lighthouse built in 1872, one of Japan's oldest still in service. Its Western style design contrasts with the sea, and it continues to guide local traffic through the strait.

The Takanosu observatory occupies the island's highest point. From there, views extend toward Awaji Island and the rocky coast below. Lawns near the trailheads provide places for picnics, with ferries crossing the water in view.

Island guides list hundreds of plant species, including subtropical ferns and coastal pines. Jagaike, also known as Snake Pond, is a wetland on the western side where seasonal plants grow.

Summer insects fill the woods with sound, while tree cover provides welcome shade. Morning fog can thread through the forest, adding another reason to spend time beyond the brick corridors. The island's appeal includes its coastal vegetation and open viewpoints as much as its military remains.

Pilgrimage routes, standard time and a stop in Kada

Tomogashima also marks the beginning of the Katsuragi pilgrimage, associated with Shugendo ascetic practice. Remains of training grounds and waypoints add a spiritual layer to a landscape better known for its fortifications.

Near the coast, the Old Navy Listening Office is built into a cliff. Windows face the sea, and skylights send light onto the floor. Its setting recalls another part of the islands' military role: listening for engines and watching for silhouettes on the horizon.

Meridian Square marks 135 degrees east longitude, the meridian that defines Japan Standard Time. Families can stand with one foot on each side of the line. The marker connects an island walk with the shared time used for daily routines across Japan, including train timetables and television broadcasts.

Kada, the fishing town where the ferry journey begins, offers other stops before or after the crossing. Awashima Shrine is known for its doll collection and the Hina Nagashi doll floating rite.

The candy pink Medetai Train on the Nankai line makes the approach part of the excursion. Some travelers finish with a soak in hot springs near the port before returning to Osaka. Together, the town and islands combine railway travel, coastal scenery, religious traditions and the remains of a once restricted military site.

Highlights

• Battery No. 3 was completed in 1892 and is the largest of Okinoshima's five surviving batteries.
• The Yura Fortresses guarded the Kitan Strait approach to Osaka Bay. Battery No. 3's artillery had a reach of roughly 7.8 kilometers.
• Red brick galleries, ammunition magazines, soldiers quarters and former gun positions remain accessible in permitted areas.
• Tomogashima opened in stages after the war and became part of Setonaikai National Park.
• The ferry from Kada Port to Nonaura takes about 20 minutes. Adult return fares are often 2,200 yen, with child fares around 1,100 yen.
• A walking loop of about 3.3 kilometers usually takes two to three hours, with extra time needed for the tunnels.
• Camping requires advance online booking at Minami Taramizu. There is no electricity, drinking water or rental equipment.
• Damaged ruins, venomous mamushi snakes, slippery paths and storm closures require care. Follow barriers and local signs.
• Other stops include the 1872 lighthouse, Takanosu observatory, Snake Pond, the Old Navy Listening Office and Meridian Square.
• Check ferry schedules and the last return sailing before setting out.