Mori Castle, Mountain castle in Ōmachi, Japan

Mori Castle is a flatland-mountain castle (hirayamajiro) on a peninsula on the north shore of Lake Kizaki in Ōmachi, Nagano Prefecture, with the lake to its north and east and marshland to its west. The site is now laid out as castle ruins, with the honmaru enclosure holding the Nishina Shrine and a monument to Nishina Moritō, and the Abé Shrine standing on lower ground at the castle's northern end.

Mori Castle was built on a peninsula on the north shore of Lake Kizaki, and some scholars link its origin to the ancient Abe clan. In the late Heian period the Nishina clan ruled the Azumi district from nearby residences, using Mori as a rear base. In 1233 Kiso Yoshishige, acting for the Kamakura shogunate, attacked the castle and expelled its lord Abe Sadataka. Takeda Shingen later placed his son Nobunobu here and had the defences rebuilt.

The name connects to the family that once governed the site. Today visitors can observe the remaining stonework and imagine how people lived and worked in this mountain fortress.

The hilltop location becomes slippery when wet, so sturdy shoes are advisable for walking the grounds. Allow time to walk through the ruins and trace the layout of the fortifications.

The castle's north and east sides were bounded by Lake Kizaki and its west by marshland, using a peninsula-shaped terrain. The honmaru site holds a small Ina Shrine built in the Taishō era, honouring the Ina clan and war dead from the Meiji period onward, with an old mound and a monument to Ina Moritō nearby.