斎藤氏
The origins of the Iwabitsu Castle are uncertain, but it is believed to have been built by the local Agatsuma clan sometime in the 13th century. During the Muromachi period, it was held by the Saitō clan, another local warlord. However, in the early Sengoku period, it was captured by Sanada Yukitaka, who had expanded from Shinano Province into northern Kōzuke Province with the backing of his overlord, Takeda Shingen. The Saitō clan had turned to Uesugi Kenshin of Echigo Province for assistance, but due to the distance from Echigo and the stratagems employed by the Sanada, the castle fell in 1563.