Odano-jo Castle Ruins are the ruins of a mountain castle found in the Nishioshimo area along Chichibu Urakaido Road. The area is home to place names that are deeply related to samurai warriors and battles, such as Nishigosho Tateato, Gaishin Yashiki, Daimon-mae, Kidoguchi, Shajo, and Kaneyashiki. According to Kai Kokushi (Official Records of Kai Province), a compilation of regional topographical information published in the late Edo Period (1603-1868), Yasuda Yoshisada, a member of the Kai Genji Clan, made Mount Odano his fort. The Kai Genji Clan flourished from the late Heian Period (794–1185) to the early Kamakura Period (1185–1333).
In later years, Takeda Nobumori (?–1455) inherited the reigns of the Kai Takeda Clan after his predecessor Nobushige died and became the fifteenth head of the clan, but it is said that he ruled for a mere five years. This is because around this time, the Atobe Clan wielded power in Kai Province as a Shugodai, or representative of the provincial governor. The Atobe Clan was related to the Kai Genji Clan and was a branch family of the Ogasawara Clan. Takeda Nobumasa (1447–1505), who succeeded Nobumori as the sixteenth head of the Kai Takeda Clan, battled and won against an army of the Atobe Clan in 1464 and hunted down the head of the clan Atobe Kageie to Odano-jo Castle, forcing Kageie to commit suicide by harakiri (Battle of Yukarizawa). By eliminating Atobe Kageie, Nobumasa succeeded in restoring the power of the Takeda Clan in Kai Province.