Ono castle (Onojo) ruins, Fukuoka.

I live only 15 minutes by car away from Onojo.  The mountain , Mt Shioji is easily seen from where I live as it dominates the eastern side of Fukuoka city.  I have visited Onojo ruins twice now.  The first time was an unscheduled visit for just a short time and the second time a few weeks ago with a Japanese castle fanatic named Adam, a pom, who currently lives in Nagano.  He is a wealth of information about all castle types and has been to over (I think) 300 castle sites throughout Japan.  We had a great day together exploring Onojo and his write-up and details about Onojo is better than I can do so he has allowed me to use his words.  So let’s have a look at Onojo ruins.

Chikuzen-Ōnojō (Dazaifu)  筑前大野城 [大宰府]

Although the first kodai-yamajiro (Ancient Mountain Fortress) I visited was Yashima-no-Ki in Shikoku, most of the oldest and grandest sites being located in Kyushu visiting Chikuzen-Ōnojō was almost entirely outside my scope of experience. Most interestingly if I had visited mountain citadels in Korea, then I might’ve been more familiar with Ōnojō. The similarities between Ōnojō and old Korean sites are considerable, and this pertains to the history of the citadel mount. Long before the rise of the mighty samurai (from the Late Heian Period) who built medieval mountain fortresses, a still yet older culture of the Yamato built huge fortified mountain citadels by piling earth and stone into ramparts ringing mountains. Called Kodai-yamajiro, Ōnojō is typical and foremost amongst these. Essentially the mountain peaks were ringed with ramparts to protect structures within a depression between the fortified peaks. The ruins of some seventy of these structures, including their foundation stones, can be seen today. The remains of ramparts, some of them clad with piled-stone in a very ancient style, can be followed as it undulates across 8km of mountainous terrain. Of what remains of the ishigaki (stone-piled ramparts) at Ōnojō it is like no other I have seen at later Japanese castles, and sweeps up mountainsides and dips into ravines, creating a smooth parapet line which follows the natural contours of the elevation. The ishigaki is found in clusters here and there. We visited four areas with ishigaki, three of which were to the north of the castle ruins. These were the Hyakken’ishigaki, the Kitaishigaki, the koishigaki and, in the south, the Ōishigaki, this being the hardest to locate because we accidentally followed a ridge instead of the dorui (earthen embankment) at one point, taking a wrong turn and having to retrace our steps. Aside from the Hyakken’ishigaki, these stone-pilings seemingly were built at points between mountainsides where streams flowed through. The Hyakken’ishigaki, that designation referring to its great length, is quite different, starting at the base of a slope, surging up it and then along the top of a mountain ridge. It is part of one of two inner ramparts which created separately enclosed areas in both the north and south of the citadel.

Whilst the circular route following ramparts which bob up and down across the mountain’s peaks and ridges can probably be walked in a few hours, to inspect everything at Ōnojō would likely take a whole day. Our route took us to (尾花) an area in the south where the remains of azakura (storehouses) can be seen. Then we (myself and Kyushu-based history blogger, Stuart Iles) stopped at a central parking area before walking a circuitous route between the three northern ishigaki remnants mentioned above. Coming back down the mountain we had just enough time to go and find the Ōishigaki in the south. So we saw some major features this way but did not have the time to walk around the whole site or inspect all of the building remains or ishigaki segments around.

Stuart and I also saw the Mizuki, a long, plain-spanning embankment built at the foot of Mt. Ōno, and an unrelated Sengoku Period fortification, Iwayajō, which is built within the ruins of Ōnojō, which says a lot about the age and size of Ōnojō that it has the ruins of a smaller, more recent castle within it! Stuart had been to these sites before and I appreciate his taking me to them and showing me around. I learnt a lot! (Stuart’s site: https://rekishinihon.com/about/)

Kodai-yamajiro is a broad term, which is why I apply it here as a catch-all, although Ōnojō can also be described as a Chōsen-shiki Yamajiro (Korean-style Mountain Fortress). Kodai-yamajiro are generally split between Chōsen-shiki Yamajiro and another group of sites, these verging upon the truly ancient, called Kōgoishi-shiki Yamajiro. The relation of the one to the other is not entirely clear and some scholars consider the Kōgoishi sites to predate the Chōsen sites. The question is whether the construction of Kodai-yamajiro date to the defeat of Baekje and the subsequent defense of Yamato in the 7th century, or whether there was an existing tradition of the construction of such structures prior. This is not a settled question. I will say that whilst all Kodai-yamajiro appear to date to the Asuka Period, the Japanese had been using large stones to line their monuments from the Kofun Period, but any link there is utterly conjectural. The extent of the involvement of Korean refugees and immigrants in the construction of sites like Ōnojō can be a thorny topic of discussion since it pertains to the identity of ancient Japanese and their relations with continental peoples.


Ōnojō was built in 665, likely by or under the guidance of the Baekje nobility who had fled Korea following their defeat at the hands of the Tang-Silla Coalition. The Yamato had dispatched troops and ships to aid the Baekje in their war and after suffering heavy losses at the Battle of Baekgang were forced to contend with the possibility of the invasion of Japan by the wrathful Tang. Ōnojō was the centrepiece of a network of fortifications surrounding Dazaifu, the political center of Kyushu (with other sites including Mizu-ki, Shōmizu-ki, Kiijō, Ashikisanjō, the Sekiya Dorui and Dazaifu’s administrative palace itself). The anticipated attack never came, however, and, by degrees, Ōnojō was abandoned to nature. As an old citadel to retreat to in times of war, it likely also played a part in the 13th century Mongol invasions of Kyushu.


Text by Adam, photos by Stuart.

Cheers mate.