Yōgaiyama Castle (要害山城, Yōgaiyama-jō) was a Sengoku period yamajiro located in Kai Province (present day Yamanashi Prefecture), constructed in the 1520s by the Takeda clan. Since 1991, the ruins have been protected as a National Historic Site since 1938. The castle is also known as the Sekisuiyama Castle (積翠山城, Sekisuiyama-jō)