The Toyama clan had ruled southeastern Mino Province since at least the Kamakura period, but Naegi Castle itself dates to 1532. Toyama Naokado, the second son of Toyama Kagetomo, lord of nearby Iwamura Castle, built the fortress on a strategic mountaintop overlooking the Kiso River. The family was deeply entangled in the politics of the age. Kagetomo had married Otsuya no Kata, the sister of Oda Nobunaga, Japan's most ambitious warlord. In an effort to prevent Takeda clan expansion into Mino, Nobunaga arranged for Naokado's daughter to be adopted and married to Takeda Katsuyori. The diplomatic marriage failed to hold. When Iwamura Castle fell to the Takeda, Nobunaga turned his strategic attention to Naegi, declaring it the most important fortification defending Mino against the Takeda advance.

The assassination of Oda Nobunaga at the Honnoji Incident in 1582 threw Japan into turmoil and Naegi Castle changed hands. Mori Nagayoshi, a general serving Toyotomi Hideyoshi, seized the fortress and installed Kawajiri Hidenaga as its castellan. The Toyama clan, dispossessed of their ancestral stronghold, fled south to Hamamatsu and entered the service of Tokugawa Ieyasu. They would wait eighteen years for their chance to return. That chance came at the Battle of Sekigahara in 1600, the decisive conflict that determined Japan's future. Kawajiri Hidenaga chose the losing side, joining the Western Army under Ishida Mitsunari. He was killed in battle. Ieyasu dispatched Toyama Tomomasa, the son of Naegi's former castellan, to reclaim the castle. Tomomasa succeeded, and Ieyasu confirmed the Toyama clan as daimyo over their ancestral holdings, establishing Naegi Domain under the new Tokugawa shogunate.

From 1600 until the Meiji Restoration in 1868, the Toyama clan held Naegi Castle through twelve unbroken generations. The domain was small by the standards of the era, but its mountain fortress was formidable. The castle exploited the natural terrain with ruthless efficiency, incorporating enormous natural boulders into its stone walls and defensive structures. The reddish clay that gave the castle its Akakabe nickname was a practical choice dictated by local geology, but it made Naegi visually distinctive among Japan's hundreds of castles. By the time the feudal system was abolished in 1871, the domain had accumulated massive debts. The castle was dismantled and its timbers and furnishings auctioned off to help pay creditors. What the auctioneers could not sell -- the stone walls, the massive boulders, the mountaintop itself -- remained, and it is these bones of the old fortress that visitors explore today.

The castle ruins sit about seventeen minutes by car from Nakatsugawa Station on the Chuo Main Line. The approach winds up through forest to a parking area, from which a walking trail ascends to the summit. Along the way, stone walls emerge from the hillside at unexpected angles, their courses interrupted by boulders too large to move. The Sannomaru, or third enclosure, opens onto views of the surrounding mountains. Higher up, the restored strut-work of the main keep crowns the summit like a skeletal frame, offering unobstructed views of Nakatsugawa city, the Kiso River snaking through its valley, and on clear days the peaks of the Central Alps beyond. The adjacent Toyama Historical Museum provides context for the castle's long history. The site rewards those who appreciate the spare beauty of ruins -- there are no rebuilt towers or painted walls here, only weathered stone and mountain air and the sense of standing where twelve generations of a single family watched over their domain.