Oyama Castle was located in Yoshida-cho, Haibara-gun, Shizuoka Pref. From the Muromachi period (1336-1573) to the Warring States period (1493-1573) there was a fort built by the Imagawa clan at this place. In 1568, Takeda Shingen rebuilt the fort as the base to capture Totomi province (present-day the western part of Shizuoka Pref.). Then in 1571, the fort was improved into a castle named Oyama Castle by Baba Nobufusa under the order of Shingen. After the construction of the castle, the Takeda clan and Tokugawa Ieyasu fought fierce battles in this area and finally the Takeda clan, preparing themselves for their defeat, set fire on the castle and fled to their home country. The site of the ancient castle was arranged into Nomanjiyama Park with the newly constructed donjon, “Oyama Castle Observatory.” The ruins of the triple moats and things pertaining to the Takeda clan, which are displayed in the observatory, remind us of the days in war-tone eras.