Kinojo Castle

Ki Castle (Kinojō) is an ancient Japanese mountain fortress located atop Mount Kinojō in Sōja City, Okayama Prefecture. It belongs to the kogoishi-style of mountain castles. Although long abandoned, parts of its stone walls and the west gate have been reconstructed, and the site has been developed into a “Historic and Natural Park” open to the public. In 1986, Mount Kinojō (including the castle ruins) was designated a National Historic Site, and in 2006 it was selected by the Japan Castle Association as one of the “Top 100 Japanese Castles” (No. 69). It is also widely known as the stage for the legend of Prince Kibitsuhiko’s battle against the ogre Ura, a tale considered the origin of the Momotarō legend.

The castle’s construction date is not recorded in historical texts, but archaeological excavations suggest it was built in the late 7th century by the Yamato Court as part of a series of defensive strongholds along the Seto Inland Sea. The site sits at an elevation of 397 meters, with walls encircling the mountain between the 7th and 9th levels, stretching about 2.8 km in length and enclosing an area of roughly 30 hectares. The walls, mainly constructed with rammed earth, featured four gates, corner towers, and water gates. Notably, the use of fuki-ishi (protective paving stones) to reinforce the walls was discovered here for the first time in Japan, demonstrating advanced engineering techniques.

Excavations revealed remains of raised-floor storehouses, administrative buildings, smithing workshops, and other structures. Artifacts such as Sue ware, Haji ware, and iron tools were unearthed, indicating that the castle supported food storage and military production. Additional earthwork remains at the southern foot of the mountain may have been used to block access routes, similar to the water fortifications of Ōno Castle in Kyushu.

Kinojō is one of the few ancient mountain castles in Japan that remain relatively well-preserved. The reconstructed west gate and corner tower, along with the “Kinojō Visitor Center” and observation decks, allow visitors to explore the ruins while enjoying panoramic views of the Sōja and Okayama Plains, the Seto Inland Sea, and even Shikoku’s Yashima Castle in the distance.

As for the name, “Kinojō” (鬼ノ城) is not directly related to the Japanese word oni (“demon” or “ogre”). In the Baekje language, “Ki” meant “castle,” while “-no-jō” in Japanese is a redundant expression. Over time, the character “鬼” (oni) was applied by association and linked to the legend of Ura, gradually giving rise to the modern name.