National Historical Site, Oko Castle Ruins

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Oko Castle Ruins

Oko Castle Ruins are located in Okoyama, a mountain in Oko-cho Yahata in Nangoku City. It is one of the representative castle ruins in Kochi Prefecture and part of it was designated as a national historical site, “The Oko Castle Ruins” on July 28, 2008. These are the ruins of the residential castle of Chosokabe, a military commander on Shikoku Island in the period of Warring States.


Aerial photograph: Oko Castle Ruins

Relief map:Oko Castle Ruins

There are several hillsides on the Kacho Plain. The serpentine Mt Oko (mountain) is one of them. 97m in altitude, the mountain is steep in its east and south sides. The north side is slightly less hilly and west side slopes in a continuous grade to 49m. The Kokubu River runs at the foot of the south side, and the west and south sides are swampy. It seems that the swamp was there at the time the castle was built, offering natural protection. It seems that the north side was connected to the Shikoku Mountain district by a low ridge, but it is now separated by a prefectural road (old national road). So, with the construction of roads toward the top of the mountain to construct a park, the castle ruins were forced to change.

The excavation of the Oko Castle ruins was conducted as a part of surrounding environment adjustment, along with construction of the Kochi Prefectural Museum of History. In an earlier confirmation survey for prefectural ruins adjustment, the first to fifth excavation study was conducted from 1985 to 1989 (government subsidy) and the sixth excavation study was conducted in 1990 by the Kochi Prefecture Education Board.

Research was conducted at the central castle areas, with the secondary, tertiary and fourth concentric areas. At the central area, paved stone remains and building’s cornerstone ruins and remains of a land blessing event were found. In the tertiary area, earthwork/stone bonds, cornerstone building ruins and stair-like remains were confirmed. Additionally, a lot of Haji pottery and imported ceramics, as well as kawara roof tiles with engraved era names were found.

The excavation was carefully conducted so the current condition of the castle ruins would not change while cutting down and planting of trees. The unearthed condition was restored by filling the remains with earth.
It is unknown when Oko castle was built. Historians think the castle existed until around 1588, when Motochika Chosokabe moved to Otakasa (or Otakasaka) Castle. Its history includes the latter 15th century and the time around 1508 to 1509, when it fell to the enemy (during the time of Motohide Chosokabe).

Walking Around Oko Castle Ruins
Wooden Statue of the Sitting Figure of Motochika Chosokabe

Designated by prefecture
Wooden Statue of the Sitting Figure of Motochika Chosokabe
Owned by Hada Shrine in Kochi City

Family Emblem of Chosokabe

Family Emblem of Chosokabe
Wooden Sorrel Oxalis in a Circle

The 21st Chosokabe Family Head
Motochika Chosokabe
Motochika Chosokabe was born in1539, the first son of Kunichika Chosokabe, in Oko.
Kunichika steadily expanded his domain thanks to his mastership of both military and diplomatic tactics. However, his sudden death by illness during a fierce battle with the Motoyama clan for the hegemony of the central Tosa region, meant that Motochika immediately inherited and became the 21st family head of Chosokabe. Following his father’s lead, he took Asakura Castle from Motoyama clan. Then, he made the Motoyama, Aki, Tsuno and other powerful clans submit to his command or be subverted one after another, finally unifying the Tosa region, which was the long time hope of the Chosokabe family.

Unearthed Article

Central Castle Cornerstone Building Remains, Paved Remains (Reproduction)

Central Castle Cornerstone Building Remains, Paved Remains (Reproduction)

From the castle ruins, pieces of imported ceramics, including celadon, white porcelain and dyed porcelain pieces were excavated along with pieces of kawara tiles and Haji pottery. Also, pieces of Japanese pottery, including Seto, Bizen and Tokoname, as well as coins, bronze and iron products were excavated. Kawara tiles with their eras engraved, bullets of arquebus hackbut, remains related to religion, iron products and coins are also included in the excavated materials.


Unearthed Article
Kawara tile with the inscription “Tensho 3” (1575)

Kawara tile with the inscription “Tensho 3” (1575)

The letters on these Kawara tiles are evidence of a large-scale remodeling of the castle in 1575, the year Motochika Chosokabe began dominating the Tosa region.

Dog-shaped Clay Model

Dog-shaped Clay Model

Bullet

Bullet

Inquiries

Kochi Prefectural Museum of History
1099-1 Oko-cho Yahata, Nankoku City, Kochi Prefecture 783-0044
TEL: 088-862-2211 FAX: 088-862-2110

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Kochi Prefectural Cultural Foundation,
Kochi Prefectural Museum of History

〒783-0044
1099-1 Oko-cho Yahata,
Nankoku City, Kochi Prefecture
TEL: 088-862-2211
FAX: 088-862-2110