Imagawa clan
The late Hōjō clan was based at Odawara Castle in far western Sagami Province and only a few kilometers from the border with Suruga Province ruled by the Imagawa clan. The Hōjō and Imagawa maintained an alliance; however, after the fall of the Imagawa clan, Suruga was occupied by the Takeda clan of Kai Province and the Hōjō territories were threatened. This threat did not diminish after the Takeda clan was destroyed by an alliance between Oda Nobunaga and Tokugawa Ieyasu, both of whom were at odds against the Hōjō. Yamanaka Castle was built by Hōjō Ujiyasu in the Eiroku era (1558–1577) to guard against this threat.