Ôiwajō is said to have been first constructed in 1193 by the Suda Clan. The Suda, vassals of the hegemonic Murakami Clan, controlled the castle. In the Sengoku period Takeda Shingen invaded, pushing out the Murakami, and the Suda Clan was split between surrendering to or resisting Shingen. The clan fractured into two camps and the Ôiwa-Suda chose to resist the Takeda. Takeda forces would eventually conquer Ôiwajō. The Ôiwa-Suda, who had since sided with the Uesugi, returned to their homeland in 1582 (after the fall of the Takeda Clan), but at that time Lord Mitsuchika set-up in Kaizujō (Matsushirojō).
The castle was nonetheless repeatedly occupied and renovated throughout the Sengoku period as various lords fought over control of the area. The first lord of the castle after the splitting of the Suda Clan, from which time the castle is confirmed to have been in place, was Suda Mitsukuni. Suda Mitsuchika would take over from his father, and it is believed he participated in the gory campaign at Toishi Castle (Murakami side). Suda Mitsuchika defended Ôiwajō from Takeda forces, but was forced to flee to Echigo, fighting for Uesugi Kenshin thereafter. When the castle fell to the Takeda is not known for sure but it was probably in 1557 with the surprise assault and brutal capture of Katsurayamajō. There were five battles fought at the castle from 1553 with the fall of Katsuraojō and the flight of Murakami Yoshikiyo to Echigo. The amount of fleeing warrior clans absorbed into Echigo by the Uesugi is crazy, and must’ve represented the sudden migration of a large number of people. Anyway, Ôiwajō is thought to have been rebuilt by Takeda forces after they took it over. I don’t know whether it was used after 1582. By the Edo period it was abandoned.
One castle blog I read speculated that Fukushima Masanori may have renovated and made use of Ôiwajō (from 1619). I found no evidence of anything dating to the Edo period at the site, however, and, considering what thin ice Masanori was on (see Takaino Jin'ya), I highly doubt he would’ve received permission for or carried out in secret the building of a new fort. At any rate, it would’ve been of little use to him.
One does not simply walk to Ôiwajō (insert Boromir) – one crawls – as many unprepared castle explorers have found out. But Nagano castle explorers are made of sterner stuff! After going back and forth looking for a proper trail to this site I realised, of course, that there weren’t any. It’s a free climb up the mountain to get to the ruins of Ôiwajō. Things get easier once one mounts the ridge, though the ridge is very rocky and steep, so that even though it is easier to climb than a muddy slope, it is actually more dangerous. The views alone are worth it though! And one may as well enjoy the view on the way because the ruins themselves are too forested to see anything from. Ôiwajō is found after climbing up and along the rocky ridge (I came from the northeastern ridge spur which has a gate in the animal barrier; I found no other way onto the mountain, and nor is there a trail up to Tsukioijō either).
The layout of Ôiwajō is like some kind of centipede with long antennae. It is anti-symmetrical. To see most of the castle without going back and forth (which is up and down), attack from the northeast, as I did. Coming from this direction one comes to a lower detached bailey. Then there is a climb up a fierce, rocky ridge. One will see a mound along the ridge after the pinnacle rocks. This mound is a bōrui, a defensive bulwark of mounded earth. A tree grows on it now. The castle ruins begin in earnest from here. There is a trench ruin and, after another steep segment, terraced baileys. Here the castle splits in two as the northwestern spur was also fortified, though there is much less to see there so I didn’t go because I didn’t want to re-climb half the mountain again.
Going south from the fork one comes to the integral baileys of Ôiwajō. Three large baileys are divided by horikiri (trenches). The middle bailey has a sub-bailey running beneath to the west. To the rear many stones are laying about, and it seems clear that they were originally piled to clad ramparts; these blocks have helped the rear of the middle bailey retain a sharp shape like, but have mostly collapsed now and are scattered about.
After the rear bailey is a huge trench as deep as a house is tall. There are some more trench complexes along the ridge thereafter and then the mountain rises sharply (and cruelly) on its way to the ruins of Amabikijō. There are some ruins on the western slope of the castle mount too but I didn’t check them out because ‘Iki wa yoi-yoi; Kaeri wa kowai’ – that’s why (but I ended up being very short on time so it’s well that I skipped them anyway).
By the way, this site’s narrow ridges were also in part sculpted, in places exposing bedrock. This feature is called ‘ikkigake (一騎駈け)’, or ‘the single knight canter’, because the narrow paths with steep drops on each side force attackers to proceed single file. A defending force can then deal with enemies a man at a time, neutralising any advantage that a larger force may’ve enjoyed.
|English Name||Oh'iwa Castle|
|Castle Type||Fortified Manor|
|Castle Condition||Ruins only|
|Designations||Local Historic Site|
|Historical Period||Pre Edo Period|
|Artifacts||Horikiri, Kuruwa, Koshikuruwa, Bourui, &c.|
|Features||trenches, stone walls|
|Access||Nearest station is Suzaka Station on the Nagano Line|
|Visitor Information||24/7 free; mountain|
|Time Required||80 minutes|
|Location||Suzaka, Nagano Prefecture|
|Coordinates||36° 39' 27.90" N, 138° 20' 35.95" E|
|Added to Jcastle||2022|
|Admin Year Visited||Viewer Contributed|
|Friends of JCastle|
|Shiro to Kosenjō|