During the warring states period (around the mid-1600s) the Nanbu clan castle located in Nanbu town was lost in a fire. After the fire, the clan decided to construct their new castle at a strategic site on top of a hill in Sannohe. This castle too was eventually lost, but today the grounds of the Sannohe castle are now a park called Shiroyama Park. There, the remains of the stone walls and moat of the Sannohe Castle Ruins can still be seen.

Also inside the Shiroyama (lit. Castle Mountain) Park is the Sannohe Museum of Local History and Culture. The building is modeled after a typical Japanese castle, making it a scenic and quaint addition to the park. The facility has interesting artifacts and information about the Sannohe area dating from the Jomon period up until the present day. The park also features a deer park that is very popular with children, event spaces, walking paths and more, making it an ideal place to have fun with the whole family. The park is particularly beautiful in spring as 1,600 cherry blossom trees bloom, making it a premier destination for cherry blossom viewing.