Mt. Iwabitsu

Mt. Iwabitsu is 802 metres of broken rock rising straight out of the Agatsuma valley, with a sheer south face that makes it look considerably bigger than it is. On its slopes are the earthworks of Iwabitsu Castle, a Sanada stronghold — this was Sanada Masayuki's mountain fortress, and it is where the young Sanada Nobushige, later famous as Yukimura, is said to have spent part of his childhood. The castle was one of the three great mountain castles of the Kanto region and never fell to assault.

What survives is not buildings but terrain: cut ridges, dry moats, and the bailey platforms, reached in about fifteen minutes from the trailhead. Continuing to the summit turns the walk into a real scramble with chains and exposed rock in places, and the reward is a view down the whole valley toward the Agatsuma river. The mountain is well known within Gunma and to castle enthusiasts, and essentially unvisited by anyone else.

Iwabitsu belongs to a category of fortification the Japanese call yamashiro, mountain castles, which relied on terrain rather than walls for their defense. Rather than building stone ramparts, defenders of a yamashiro shaped the mountain itself — cutting artificial ridges to slow an advancing force, digging dry moats across natural approach routes, and terracing hillsides into stepped platforms that gave defenders higher ground at every stage of an assault. Because these earthworks used the mountain's own material and contours, many yamashiro sites like Iwabitsu survive today as recognizable terrain even though every wooden structure that once stood on them is long gone.

The Sanada clan, who held Iwabitsu, built a regional reputation during Japan's Warring States period for using mountain terrain and clever tactics to hold off far larger forces, a reputation later cemented by their role in one of the era's most famous final battles. Their base in this stretch of Gunma — then a contested border region between rival domains — put them in a strategically important but perpetually exposed position, which is part of why the family invested so heavily in a network of mountain strongholds like Iwabitsu rather than relying on a single fortified base.