His sixth generation descendant, Sōma Shigetane (相 馬 重 胤), built a fortified house at Odaka (小 高) in 1326, which was later expanded into a castle.

The Inner bailey (本 丸) of the castle was built on a hill that rises above the local plain between the Koizumi and Udagawa rivers.

In the southwest corner of the Inner bailey was a three-story donjon (天 守, marked in red on the map), which was lost in 1670 by a lightning strike.

In the southeast was a separate area once containing the residence of the daimyō (marked "N" on the map), which is now occupied by the Nakamura Jinja, a Shinto shrine dedicated to the Sōma clan ancestors.

The defences of the castle were the strongest to the north, where the moats could be cut to flood a large area, turning it into an impassable swamp.