Kokuzousan Castle (Aida)

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AidaKokuzousanjou (33).JPG


Aida Castle was originally the main base of the Aida Clan in the Kamakura period. The Aida were descended from the influential Unno Clan, and in the Sengoku period aligned with the Ogasawara Clan under Ogasawara Nagatoki with whom Takeda Shingen was warring. The Aida Valley was an important route into northern Shinano, vital to Shingen’s ambitions to conquer the province. The Aida initially resisted his advances, but Shingen invaded the valley in 1553 and burnt Akiyoshi Fort to the ground, forcing the clan to surrender. Later that year Takeda forces lost important battles at Fuse and Sarashina during the first Kawanakima campaign to Uesugi Kenshin’s forces. Uesugi forces went on the offensive, attacking Aoyagi Castle. They then attacked forts in Aida as well.

After the fall of Takeda Katsuyori and Oda Nobunaga in 1582, the Ogasawara would reclaim parts of Shinano, but not without resistance. The Aida resisted the Ogasawara, their old masters, with aid from the Uesugi, but they were defeated by Ogasawara Sadayoshi. Sadayoshi may then have renovated the fort complex for use as a base in pressing northward into the Uesugi resurgency there. Refortifying Aida would’ve been crucial in protecting Fukashijō (Matsumoto Castle).

See also: Aida Yakata

Visit Notes

There are many fortification sites called ‘Kokuzōsan’, which appears to be a popular name for mountains in Shinano, and the province / prefecture has at least five. Two are actually part of very large complexes of fortifications, and Aida-Kokuzōsanjō is one of those, part of the Aida-jōkangun (会田城館群), a group of fortification and residence sites in and above the valley of Aida. Of that group, Aida-Kokuzōsanjō is not the largest of the fortifications, but it may be considered the main one, as it sits atop of all the others on Mount Kokuzō, and is also referred to as Minenojō (峯ノ城), or ‘castle at the peak’.

The entire complex consists of many smaller fortified ridges and terraced areas, but the main clusters of fortifications are generally considered to be Kokuzōsanjō, Nakanojinjō (Nakanojin Castle), Utsutsujō (Utsutsu Castle), and Akiyoshi-toride (Akiyoshi Fort). These ones often get dedicated profiles on castling blogs, but there are other fort sites besides. Also, Karatoriyajō (Karatoriya Castle) on a neighbouring mountain is also sometimes included. The complex is the core of a network of fortification sites all around and throughout the valley, in fact.

Aida-Kokuzōsanjō itself is made up of narrow cleared spaces which form baileys along the ridge divided by trenches cut into the earth and rock. The view from the topmost bailey complex is incredible, and it’s nearly impossible to imagine assaulting such a hard to access castle. To the south, on the mountainside, there are columns of terraced pocket baileys which climb up the mountain’s frond-like ridges like gigantic stairways. I descended via the longest column of terraced baileys which start beneath the Iwaya-jinja, a shrine built clinging to a massive boulder. To the west of the main ridge is a jagged row of rock rising from out of the mountain like the spiky spine of a dragon. The best views can be enjoyed here. Beneath here, further along the ridge, is a well ruin. The ridge drops swiftly in elevation thereafter. Following that ridge brings one to several detached baileys and, eventually, the site of Utsutsujō. The other aforementioned forts are located to the southwest. Akiyoshi-toride has immediately beneath it an area of extensive, wide terracing and stone walls which probably hosted residences or barracks.

  • Stone wall and possible gate site
  • Horikiri (trench)
  • Castle mount from below
  • main bailey and castle marker

Castle Profile
English Name Kokuzousan Castle (Aida)
Japanese Name 会田虚空蔵山城
Alternate Names Aida-Minenojō
Founder Aida Clan
Year Founded Kamakura Period
Castle Type Mountaintop
Castle Condition Ruins only
Historical Period Pre Edo Period
Artifacts Horikiri, Kuruwa, Ishigaki, &c.
Features trenches, stone walls
Visitor Information
Access Bus from Matsumoto Station
Visitor Information 24/7 free; mountain
Time Required 120 minutes
Location Matsumoto, Nagano Prefecture
Coordinates 36° 22' 3.40" N, 138° 0' 35.57" E
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Added to Jcastle 2023
Contributor ART
Admin Year Visited Viewer Contributed
Friends of JCastle
Jōkaku Hōrōki
Shiro Meguri

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