The location of the castle was also intended to control traffic on the Ōshū Kaidō highway from the capital to the northern end of Honshū island, and beyond to Ezo (modern Hokkaidō), and also traffic on a route across the Ōu Mountains which connected the Pacific and Sea of Japan coasts of Japan.

Because of its strategic geographic position, the area around Morioka had been regarded as the center of northern Ōshū since the construction of nearby Shiwa Castle by the imperial dynasty in the early Heian period.

His holdings were re-confirmed after the Battle of Sekigahara by Tokugawa Ieyasu, and Morioka remained the seat of the Nanbu clan until the Meiji Restoration.

As white granite was readily available in the vicinity, most of the ramparts were faced in stone, which was unusual for castles in the Tōhoku region at the time.

Both the Toyotomi and Tokugawa governments encouraged the Nanbu to build a strong fortification, partly as a counter to the strength of the Date clan to the south.

The surrounding Iwate Park (岩手公園, Iwate Kōen) (now known as Morioka Castle Ruins Park (盛岡城跡公園, Morioka-jō ato kōen)) contains a monument inscribed with a poem of Kenji Miyazawa, and also a monument in memory of Takuboku Ishikawa inscribed with one of his poems: 不来方のお城の草に寝ころびて空に吸はれし十五の心 Kozukata no oshiro no kusa ni nekorobite sora ni suwareshi jū-go no kokoro