Sunday, August 18, 2019

Iwakisan Kougoishi Castle -Mysterious ancient castle without record-

Iwakisan Kougoishi Castle

-Mysterious ancient castle without record-



Name: Iwakisan Kougoishi castle (Iwakisan Kougoishi)
Place: Shiota Hikari city, Yamaguchi
Location: 33.9874759531679, 132.0404442518039
Type: Ancient Castle
Built: 7th century?
Remaining remnants: Stone walls and clay walls 

Brief History

Iwakisan Kougoishi Castle (石城山神籠石) is located around the hilltop of Iwakisan mountain, one of about 350 meter height from hillside at the root of Murozu peninsula. At this place flatland spreads roundly from current Yanai city to Tabuse town surrounding Akagoyama mountain, and castle site can look down this flat place.

Now it becomes land by natural and artificial land reclamation, but in ancient era this flat area was a channel named Ko-Yanai Suido (ancient Yanai channel). Being surrounded whole direction by hills, it was an ideal point to anchor the ships and wait for wind. It is said that in ancient era, Kumage Kingdom prospered based on marine transportation, and huge tombs of Kings spreads around the hillside of mountain.

As Iwakisan Mountain was a sacred place of Kumage Kingdom, after the disappearance of kingdom it continued to be a religious place. At the mountain there is Iwaki Shrine, which is told as being established in 6th century. Iwaki Shrine has been one of the important shrines of Suo Province (south half of Yamaguchi prefecture), thus remnants of castle once grasped as ritual facilities.

However, along with the progress of study, it became regarded as ancient castle built by Yamato dynasty around 7th century or 8th century. The remnant has common element to ancient mountain castles built around Kyushu area and Seto-Naikai Sea such as Onojo castle, Kii castle (Fukuoka prefecture) or Kinojo castle (Okayama prefecture). 

Arise of diplomatic tension

In 663, an alliance army of Yamato Dynasty and exiled Korean Beakje kingdom ruined in 660 which attempted the recovery of the Kingdom, suffered severe defeat by alliance of Korean Silla kingdom and Chinese Tang Dynasty at the battle of Baekgang, which occurred at the mouth of Geum river in southwest part of Korean peninsula. 

Yamato Dynasty lost many soldiers and ships, and had to seriously consider the possibility of attack to Western Japan, especially north part of Kyushu island. Under such situation, Yamato Dynasty had to protect Dazaifu city, the vice capital of Western Japan its local government among others, and Seto-Naikai Sea where is directly connected to capital area of Yamato dynasty such as Osaka area or Nara area.

Build of ancient style castles

Opportunely Baekgang kingdom was extinguished and many refugees including castle engineers came to Japan, Yamato Dynasty build many Chosun style mountains castles such as Onojo castle, Kii castle (Fukuoka prefecture), Kinojo castle (Okayama prefecture), Kikuchi castle (Kumamoto prefecture), Yashima castle (Kagawa prefecture) and Takayasuyama castle (Osaka prefecture) around north part of Kyushu island and coast of Seto-Naikai sea, expected invasion rate by foreign country to the capital using coast fleet. 

Although there was no record on the construction of Iwakisan Kougoishi castle, but it was clear this castle was also built at this time. Castle site is at the western border of Seto-Naikai Sea, and Murozu peninsula projected to the sea was an ideal place to control entrance of ships to Seto-Naikai sea.

Considering invasion of enemy fleet passing Kanmon Straight, it might be used as an anchorage point of the fleet to intercept the enemy at the entrance of the inner sea. Contrary to this, once castle site was deprived, there was a possibility to be used as an ideal bridgehead of enemy fleet for anchorage and supply. Therefore it was necessary to keep castle site by any means.

Structure of castle

Same as other ancient castles, Iwakisan Kougoishi castle consists of line of clay walls encircling hilltop area and water gates which is the stone wall built at the valley. The south half of the castle utilizes horse shoe shape of the mountain which has open side toward north, and north half of the castle is artificially built line of wall.

Clay wall of the castle is about 5 meter tall and built by piling sands and pressing it. The bottom layer of the wall has single layer line of stone wall, to prevent the collapse of the wall by water erosion. The diameter and total length of stone wall is about 500 meter and 2 kilometer, and a bit smaller than other ancient castles as this castle might not have administrative function.

Water gates were built at four directions of the mountain, to protect the enemy attack from the valley and hold the water. North water gate which might be the first weak point of the castle had three line of stone walls of 5 meter height like U letter shape. Shortly apart from north water gate there was north gate of the castle, which might be main gate of the castle and there remain corner stone which has a hole to stand up of the pillar.

Western water gate has three layer of stone walls, as it might not possible to built tall one stone wall as a technical matter. Eastern water gate is smaller than north or west because of small size of the valley but has same height. Inside the line of clay wall, there are several flat areas originally used for storage or camping space but later turned to the ground of shrines and temples. 

Afterward of castle

Fortunately the invasion of enemy fleet did not realize, and the mountain ceased to be a fortress and returned to a religious place. In 15th century Ouchi clan which was the governor of Suo province donated the main hall of Iwaki Shrine and it remains until now. There also remains old gate of former Jingo-Ji temple, now used as the gate of shrine.

At the end of Edo era, the mountain was used as a headquarter and training place of Daini Kihei Tai, a militia of Choshu domain later turned to regular army. Daini Kihei Tai made achievement at the battle of the domain against Edo Shogunate, and there remains the tale of usage by the troop such as power storage or camping barracks.

Now the remnant of the castle quietly remain covered by forest, but formerly the line of clay wall and stone wall reflecting the sun light well seen from the inlet. Because of the change of the situation and terrain the role of the castle was once forgotten, but considering its structure and location it was surely a castle of Yamato Dynasty to protect a critical inlet, even though not having official record.


60 minutes walk from JR West Sanyo Honsen line Iwata station. 30 minutes drive from Sanyo-do Expressway Kumage interchange to parking at hilltop area.

Related Castles

Onojo Castle -Fortress complex protected vice capital of ancient Japan-
Kii Castle -Rear guard of fortified vice capital-
Kinojo Castle -Mysterious ancient mountain castle connected with famous tale-
Kikuchi Castle -Supply base of ancient dynasty-

Pictures (click to enlarge)

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