Iwao Castle (Saku)
Iwaojō is a hilltop fort ruin in Naruse Township, Saku Municipality. Ruins include earthworks such as baileys, dorui (earthen ramparts) and trench remains. The layout of the hill fort consisted of a ladder-like series of sequential baileys divided by karabori (dry moats), with the main bailey at the
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History
"Ôi Masayuki built the castle. Sanada Yukitaka resided at the castle. And Yoda Nobushige died at the castle."
Iwaojō was built by Ôi Yukitoshi, son of Ôi Yukiharu, in 1479. The second patriarch was Ôi Yukimitsu, who took over in 1486 and repelled an attack on the castle by the Kai-Takeda in 1489. Yukimitsu left his homeland on a religious pilgrimage and only returned after a quarter of a century, it is said, whereupon he erected a pagoda. Clearly he was a very devout man. Yukimitsu's successor was Ôi Yukimasa, who was born at Iwaojō in 1484. Maybe Yukimasa renovated the castle (and the municipal website mistakenly calls him 'Masayuki')? But the castle's later layout suggests, at least in part, that it as built with the castle-building technology of the Kai-Takeda.
In 1543, Ôi Yukiyori surrendered to Takeda forces and was vassalised by the Takeda in 1545, the Ôi Clan subsequently fighting for them. At that time Sanada Yukitaka was installed as the garrison commander and steward of Iwaojō. Takeda Shingen performed a ground-breaking ceremony at Iwaojō in 1551, suggesting that it was rebuilt at that time, no doubt incorporating Takeda castle-building technology such as the use of umadashi barbicans. It is said that Sanada Masateru and Sanada Masayuki were born at Iwaojō.
Following the 1582 destruction of the Takeda Clan, Tokugawa-alligned forces invaded Takeda territory, and Yoda Nobushige besieged Iwaojō. It was at Iwaojō where defending forces, led by Ôi Yukiyoshi, killed Yoda Nobushige with firearms as he led a direct assault from the castle's outer baileys. Despite Nobushige's death, however, and this initial success of the defenders to drive the enemy back, overwhelming force was soon applied to Iwaojō, forcing the Ôi to surrender their castle a month later in early 1583; Iwao Castle was destroyed at that time.
Field Notes
Iwaojō is a hilltop fort ruin in Naruse Township, Saku Municipality. Ruins include earthworks such as baileys, dorui (earthen ramparts) and trench remains. The layout of the hill fort consisted of a ladder-like series of sequential baileys divided by karabori (dry moats), with the main bailey at the top. The main bailey and second bailey were further protected with dorui. Between the main bailey and second bailey was no karabori, but only dorui. It seems the karabori between the main bailey and nishinomaru (west bailey) was particularly deep, and it can still be clearly seen to this day, albeit I found it rather overgrown.
Beyond the third bailey was the lower portion of the castle, and this apparently had an umadashi (circular barbican) configuration of karabori. However, it no longer remains, likely filled in for agricultural purposes. The lower castle has been developed over, but originally consisted of at least four more baileys, making the fort quite a large one.
The castle site is now a shrine, and the structure has been altered in places for this and other purposes. The stone walls seen at the site, for example, were built for the shrine. The southern face of the castle-mount features sheer cliffs, making the terrain naturally quite formidable. To the south of the castle, from whence I climbed the hill, is an old temple site. There are apparently plans to rebuild the temple hall.
| Castle Profile | |
|---|---|
| English Name | Iwao Castle (Saku) |
| Japanese Name | 佐久岩尾城 |
| Founder | Ôi Yukitoshi |
| Year Founded | 1479 |
| Castle Type | Hilltop |
| Castle Condition | Ruins only |
| Designations | Prefectural Historic Site |
| Historical Period | Pre Edo Period |
| Artifacts | Horikiri, Dorui, Kuruwa, Karabori |
| Features | trenches |
| Visitor Information | |
| Access | Nearest Station is Kita-Nakagomi Station on the Kōmi Line |
| Hours | 24/7 free; mountain |
| Time Required | 60 mins |
| URL | Castle Website |
| Location | Saku, Nagano Prefecture |
| Coordinates | 36.25394, 138.42914 |
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| Admin | |
| Added to Jcastle | 2025 |
| Contributor | ART |
| Admin Year Visited | Viewer Contributed |
| Friends of JCastle | |
| Jōkaku Hōrōki | |
| Yogo | |
