Château de Kasugajo, Medieval Japanese castle in Saku County, Japan
Kasugajo is a mountain-based fortress in Saku County built on ridges at about 180 meters in elevation, with earthen ramparts and dry moats arranged across terraced levels. The fortification system uses multiple walled enclosures connected by passages and positioned to control movement through the mountain terrain.
The Nagao clan built this fortress during the Sengoku period as a military stronghold that passed to the Uesugi clan and remained active until 1607. The site played a role in regional power struggles through multiple generations of samurai leadership.
The castle shows traditional yamashiro design with multiple defense layers built into the mountain slopes, which you can still trace when walking through the site. This arrangement reveals how people organized their fortifications to work with the natural terrain rather than against it.
You can reach the site from either Hirasaku Station on the Koumi Line or Higashi-Komoro Station on the Shinano Railway, both requiring some travel time to the mountain. The terrain is steep with uneven paths and stairs, so wear appropriate footwear and allow time to explore the different levels at your own pace.
The fortress kept its original earth-based fortification system without stone walls, which is less common than the stone castles built later in Japan. This feature shows how early military engineers adapted defenses to mountain terrain using only soil and wood structures.
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