The location of Saga Castle was originally a fortified village under the control of the Ryūzōji clan, warlords of a small area of northern Kyūshū in the Muromachi period. After Ryūzōji Takanobu was defeated by a coalition of Shimazu and Arima forces in 1584, his retainer Nabeshima Naoshige gained control of the castle. Naoshige allied the clan with Toyotomi Hideyoshi and gained personal distinction during the Japanese invasions of Korea, during which time he befriended noted castle architect Katō Kiyomasa and future Shōgun Tokugawa Ieyasu. Following the Battle of Sekigahara, the Nabeshima clan was confirmed in its holdings in Hizen province, and Naoshige’s son, Nabeshima Katsushige became 1st "daimyō" of Saga Domain. Naoshige began work on rebuilding the castle with the approval of the Tokugawa Shogunate beginning in 1602, with the work completed under Katsushige by 1611. The original structure included a five-storey donjon surrounded by a system of 80-metre wide moats. Unusually, the moats are not surmounted by stone walls, but by earthen ramparts tall enough to conceal the inner fortifications. These ramparts were also planted with pine and camphor trees for additional concealment, which lent the castle its nickname.