Sannohe Castle

Sannohe Castle was a mountain castle (yamajiro) in Sannohe Town, Sannohe District, Aomori Prefecture, also known as Tomegasaki Castle. It is said that Nanbu Harumasa, whose base, Seijuji Hall, was burned down by a retainer's arson in 1539 (Tenbun 8), built a new residence on this site. Until Nanbu Nobunao moved his residence to Kunohe Castle (Fukuoka Castle, Iwate) after the Kunohe Masazane Rebellion of 1591 (Tenshō 19), Sannohe Castle functioned as the principal castle of the Nanbu clan. The castle site was designated a National Historic Site on March 15, 2022.

Location and Structure

The castle stands on a river terrace formed by the erosion of the Mabe and Kumahara rivers. The two rivers served as natural water moats, making it a naturally defensible stronghold. The castle grounds extend about 1.5 km from northeast to southwest and about 400 m from northwest to southeast, with a height difference of about 90 m from the surrounding lowlands.

Lords and Succession

After its construction in 1539, Sannohe Castle served as the principal castle of successive Nanbu clan lords throughout the Sengoku period. It also functioned as a base for controlling the rapidly expanding retainer band and supported the growth of Nanbu power. Its role ended after the Kunohe Masazane Rebellion, when Nanbu Nobunao moved his residence to Kunohe Castle (Fukuoka Castle, Iwate), and in Kan'ei 10 (1633) Morioka Castle was officially designated as the principal residence.

Current State and Preservation

The castle ruins were designated a National Historic Site as "Sannohe Castle Ruins" on March 15, 2022, and have been developed and opened to the public as "Historic Site Sannohe Castle Ruins Shiroyama Park." Stone walls, moat remnants, earthworks, and masugata gate structures from that era can still be confirmed today.