* Image is for illustrative purposes only.Okō Castle
岡豊城
Okō Castle is a historic castle in Shikoku dating from the From around the Nanboku-chō period through the mid-16th century, when Chōsokabe Motochika expanded and fortified the castle, whose lord was Chōsokabe clan (Chōsokabe Kunichika and Chōsokabe Motochika).
Overview
Okō Castle (Okōjō) is a yamashiro (mountain castle) located in Nankoku, Kōchi Prefecture, and is known as the main stronghold of the Chōsokabe clan, who unified Shikoku during the Sengoku period. The castle was built on an independent hillock at an elevation of 97 meters and is characterized by its layout that exploits the natural fortress terrain surrounded by the Monobe River and mountains on all sides.
The Chōsokabe were originally minor lords of Tosa, but during the time of Chōsokabe Kunichika, they began to extend their influence using Okō Castle as their base. His son, Chōsokabe Motochika (1539–1599), was called a "talented person of Tosa"—a general of exceptional wisdom and valor. Beginning with his maiden battle, where he defeated the Motoyama clan at the Battle of Nagahama in Eiroku 3 (1560), he had nearly unified Shikoku by Tenshō 13 (1585). Many anecdotes attest to Motochika's military prowess, such as "Ichijō Kanetsada" and "Motochika's Hundred Warriors," and he was beloved by the people of Tosa.
The castle comprises five terraced enclosures (dankurawa) forming a full-fledged yamashiro, with the main citadel, second, third, fourth, and fifth terraces arrayed along the ridge. During the times of Kunichika and Motochika, large-scale fortification works were undertaken, with each terrace equipped with defensive features including yagura (watchtowers), stone walls, and horigiri (horizontal moats). Today the castle site is preserved adjacent to the Kōchi Prefectural Museum of History and Folklore, which displays excavated artifacts including pottery, weapons, and roof tiles.
Following Toyotomi Hideyoshi's Shikoku Campaign (1585), Motochika's domain was reduced to Tosa alone, though he subsequently participated in Hideyoshi's Kyushu Campaign, the Odawara Campaign, and the Korean Invasion. Okō Castle was abandoned when Motochika relocated his seat to Urato Castle in Tenshō 19 (1591). After Motochika's death in 1599, the Chōsokabe clan fought for the Western Army at the Battle of Sekigahara and were subsequently stripped of their domain.
Connection to Swords
The relationship between Okō Castle and swords stands at the heart of Chōsokabe clan's Sengoku warrior culture. Chōsokabe Motochika is renowned for adopting the ichiriyōgusoku (一領具足)—a unique half-farmer half-warrior system through which he organized Tosa's peasantry as warriors. Under this system, Tosa farmers equipped themselves with swords and spears alongside their agricultural tools and participated in combat. The sword transcended its role as mere weapon: it was simultaneously a symbol of samurai status and pride, and for ichiriyōgusoku soldiers, the blade served as the vessel of the soul itself. Motochika himself was known as a discerning connoisseur of swords and acquired numerous renowned blades both as war spoils and tribute. Swords confiscated from the daimyo houses of Awa, Sanuki, and Iyo—territories he conquered during Shikoku's unification—are recorded as having been stored in Okō Castle's arsenal. The swordsmiths of Tosa, celebrated as tosomono (Tosa blades), forged practical blades designed for the realities of combat. During the Korean Invasion, Chōsokabe Motochika proceeded to the Korean Peninsula under Toyotomi Hideyoshi's command, and at that time Tosa swords were brought to Korean lands as emblems of Japanese military might. The memory of the era when all Tosa peasants, bound by the ichiriyōgusoku system, carried swords remains to this day as the foundation of Tosa's warrior culture and a source of pride for the people. The Kōchi Prefectural Museum of History and Folklore displays iron goods and armor excavated from Okō Castle, allowing visitors to tangibly experience Tosa's warrior culture during the Sengoku period.
Highlights
- Five terraced enclosure ruins — the Chōsokabe yamashiro layout is preserved in complete form along the ridge
- Vista from the final tier — magnificent panorama encompassing the Monobe River and Shikoku Mountains
- Kōchi Prefectural Museum of History and Folklore — comprehensive display of artifacts excavated from Okō Castle and materials relating to the Chōsokabe clan
- Earthen ramparts and horigiri ruins — on-site experience of Sengoku-period yamashiro defensive engineering
- Bronze statue of Chōsokabe Motochika — view the hero of Tosa in commanding presence
- Site of the Chōsokabe clan's end — castle tour tracing the historical aftermath of the Battle of Sekigahara
* Opening hours and admission fees are subject to change. Please check the official website before visiting.