Kannonji Castle, Mountain fortress in Ōmihachiman, Japan

Kannonji Castle is a ruined fortress on Mount Kinugasa in Ōmihachiman, Japan, sitting at an elevation of 432.9 meters (1,420 feet). The site spreads across several terraces and ridges, with stone walls following the natural contours of the terrain and leading to different defensive positions.

The exact construction date is unknown; the Taiheiki records that Rokkaku Ujiyori held a fort here in 1335 against Kitabatake Akiie's army. In 1351 Sasaki Dōyo and the Rokkaku brothers took refuge in the castle, then called Sasaki Castle. During the Ōnin War it was besieged three times, and in 1568 Rokkaku Yoshikata and his son fled before Oda Nobunaga's advance, surrendering the castle without a fight.

The fortress follows the yamashiro style, in which defenses were built into steep mountain slopes to create natural barriers against attack. Hikers today walk through the remains of the defensive layers, seeing how warriors once used elevation to control the approach routes and repel invaders.

A roughly one-hour hike from Kuwanomi Temple leads up to the ruins, with a steep path through forest and rock. Sturdy footwear and enough water are important, as the climb is demanding and markings are few along the way.

The enclosure names at Kannonji Castle are personal names rather than numbers: Itō-maru, Sawada-maru, Mabuchi-maru, Mitsui-maru, Baba-maru, Ōmitsuke-maru, Mikuni-maru, Iba-maru, Shindō-maru and Gotō-maru. This is thought to date from Rokkaku Sadayori's time, when he settled vassals and local lords inside the castle in what is considered the first documented shirowari, or division of a castle among retainers.