This valley was the route of the Shinshu Kaidō, a road which led from Takasaki to Shinano Province via the Torii Pass, and the castle thus controlled a strategic point on this highway.

The origins of the Iwabitsu Castle are uncertain, but it is believed to have been built by the local Agatsuma clan sometime in the 13th century.

He also rebuilt the Iwabitsu jōkamachi into a fortified shukuba by surrounding the entire town with a series of forts and gates, similar to the layout which he later employed when designing the new stronghold for the Takeda clan at Shinpu Castle as few years later.

[citation needed] Following the establishment of a stable regime under Toyotomi Hideyoshi, Iwabitsu and Numata were awarded to Sanada Nobuyuki, while his father Masayuki retained Ueda Castle.

The jōkamachi survived as post station on the highway, but was relocated closer to the Agatsuma River.