Nirayama Castle located in Mt. Ryujoyama in Nirayama, Izunokoku City, Shizuoka Pref. is a flatland-mountain castle pertaining to Hojo Soun. The exact construction year is unknown, but according to Hojo Godaiki (the record of the 5 generations of the Hojo clan) it was built by Toyama Buzen-no-Kami at some time during the Bunmei era (1469-1486). In 1491, the castle was captured by Hojo Soun, who embarked on a full-scale construction of the castle. Soun lived in this castle until he died in 1519. During the Siege of Odawara in 1590, the Hojo clan protected this castle by sending more than 3,000 soldiers. After the fall of the Hojo clan, Naito Nobushige moved to this castle, but he was transferred to another fief in 1600 and the castle was abandoned. The area near the school building of Nirayama High School is thought to be the place where Soun’s residence used to be located. The ruins of Honmaru (the main castle), earthworks and dry moats can be seen in the higher part of the mountain.