During the Sengoku period, in the early 16th century, the Honjō clan fortified the top of Mount Gagyū as their residence.

The Honjō were a cadet branch of the Taira clan from the Chichibu region of what is now Saitama Prefecture and seized this area in the Kamakura period.

Kenshin defeated the Agakitashu petty lords one after another, and eventually forced Honjō Shigenaga to submit.

During the 1563 campaign against the Odawara Hōjō, he captured Sano Castle in Kōzuke and was ranked as second among Kenshin's generals.

Shigenaga eventually surrendered in 1569 and was pardoned by Kenshin, but he revolted again ten years later, this time with the backing of Oda Nobunaga.

In 1588, Shigenaga defeated a Mogami army at the Battle of Jyugorihara and extended Uesugi control into the Shōnai area.

Due to its strategic position at the entry to Dewa Province, the castle was attacked and destroyed by imperial forces.

Fearing that the remnants of the castle would disappear completely, descendants of former Murakami domain samurai formed a society to petition for the protection of the remaining ruins.

[2] Mount Gagyū is long and narrow, extending in a north-to-south direction, with the main areas of the castle spread along this ridge.