Ōno Castle

Ōno Castle (Ōnojō) is located in Fukuoka Prefecture, spanning the municipalities of Dazaifu, Ōnojō, and Umi in Kasuya District. It was constructed atop Mount Shioji (also known as Ōkiyama), which rises about 410 meters above sea level, and is one of Japan’s most famous ancient mountain fortresses. Its ruins were designated a Special Historic Site of Japan in 1953 under the title “Ōnojō Ruins,” and in 2006 it was included in the list of Japan’s Top 100 Castles (No. 86).

The origins of Ōno Castle are closely tied to Japan’s ancient defense system. Situated in the mountains directly behind the Dazaifu Government Office to the north, its walls were built along the ridgeline in a horseshoe shape, linking the valleys and forming about 6.8 kilometers of earthen and stone ramparts. In some parts on the north and south sides, double ramparts were constructed, bringing the total length to about 8.4 kilometers, making it one of the largest ancient mountain fortresses in Japan. The castle area spans roughly 1.5 kilometers east to west and 3 kilometers north to south, boasting a grand and tightly organized defensive structure.

Within the castle grounds, there were nine gates, including the Dazaifu Gate, the main entrance facing the Dazaifu Government Office. In addition, the valleys still preserve a variety of ancient hydraulic structures, such as the permeable Hyakken Stone Wall, the Mizunote Stone Wall, and the Tonmizu Stone Wall with its water intake, demonstrating the emphasis placed not only on military defense but also on the management of water resources and daily life needs.

The modern city name of Ōnojō derives from this ancient mountain fortress.