Kogo-ishi or kogo-ishi style mountain castles were castles built on mountains in ancient times, and because there is no description of them in the 'Nihon shoki' (Chronicles of Japan) or 'Zoku Nihon shoki' (the sequel to Chronicles of Japan) they can be identified only through remains. They have been identified as the surmised locations of ancient mountain castles (Korean-style mountain castles), but the process of construction and who they were built for is totally unknown.
Their characteristics are that on mountains which are 200 - 400 meters high, "kiriishi" (stones made by breaking rocks) about 70 centimeters wide, were placed from the top to the middle of the mountain, running for several kilometers, and which had a water gate when passing through a valley.
Sozaburo YAGI estimated that kogo-ishi had initially been built before the early seventh century through a comparison of the method of construction of tumulus stone chambers. Takeshi KAGAMIYAMA estimated that kogo-ishi were built after the middle of the seventh century because the gap between the holes for posts in front of the arranged stones was about three meters and nearly equal to 2.95 meters) using the "toshaku" (unit of measurement during the period of Tang Dynasty in China) which was used from the middle of the seventh century. However, using the "shoshaku" measurement used during the period of the Northern and Southern Dynasties (China), it would be 2.94 meters and be nearly equal to the above figure, so there is a question mark over the assertion that they were built after the middle of the seventh century. The location of the remains of sixteen kogo-ishi are known, stretching from northern Kyushu to the Setouchi seacoast.
Discovery and dispute
In 1898 Shojiro KOBAYASHI, in the first time it was presented to an academic society, claimed the kogo-ishi on Mt. Chikugo-Kora was 'a sacred ground, preserved and separated as a holy area.'
In 1900 Sozaburo YAGI explored the kogo-ishi located in the Kyushu region and insisted that, 'these kinds of large constructions only appear as part of castles' and after that, the dispute between the sacred-place theory and the castle theory began.
During the excavation and research of the "Mt. Otsubo" kogo-ishi in Takeo City in Saga Prefecture in 1963, the dorui (earthwork) built using the "hanchiku" construction method behind the arranged stones, and the traces of posts aligned at three-meter intervals in front of the arranged stones were discovered, and as a result it was widely accepted that kogo-ishi were mountain castles.
However, just looking around the various kogo-ishi it becomes apparent that the differences between them are many. There are those such as Goshogatani which seem to have been repeatedly reconstructed, others such as Mt. Rai which are very far from the living space and food-production areas, which could not easily provide water, were inadequate for taking refuge and unusual in their position in relation to the sacred area, and those such as Mt. Otsubo located on a small hill with an area of rice cultivation and agriculture.
It is unclear as to when the kogo-ishi were built, and even if they are of the same era it is probable that we only collectively refer to the wide variety of constructions as 'kogo-ishi' based on the distorting prism of what remains today.
The Mt. Otsubo excavation research merely determined that 'some kogo-ishi were used as mountain castles.'
In the case of kogo-ishi near to living areas, it is natural that the defining characteristics of kogoishi cannot be identified through the materials excavated from the ruins.
Also, even if all of these were simply ancient mountain castles, as they exist in a broad area of western Japan in potentially strategic positions and have been nearly unknown until now, they are historically important when considering the period around the formation of Yamato Dynasty and the process of its creation. Even though from the viewpoint of present-day (the early 21st century) historical study they may not be seen as very attractive, some say that they are very useful case studies because through them we can learn how history is formulated even in modern times.