Oshi Castle

Overview

Oshi Castle (忍城, Oshi-jō) is a Japanese castle in Gyōda, Saitama Prefecture. It was built in 1479, during the Muromachi period (1336–1573), in the eleventh year of the Bunmei 文明 era. The castle served as the seat of the Oshi Domain, a holding assessed at 100,000 koku, during the Edo period. It is best remembered for the Siege of Oshi, a major conflict of the late Sengoku period.

The castle carried two nicknames. It was called Kama-jō (亀城, Turtle Castle) and Oshi-no-uki-jō (忍の浮き城, the Floating Castle of Oshi). Contemporaries regarded it as one of the seven main strongholds of the Kantō region, a group known as the 関東七名城.

History

Oshi Castle was constructed in 1479, in the eleventh year of the Bunmei era, during the Muromachi period. Its position in the Kantō plain made it a strategic point for regional warlords. The castle’s defences relied on the surrounding marshes and rivers, which gave it the reputation of a floating fortress.

The castle’s most famous moment came during the Siege of Oshi in the late Sengoku period. The attacking forces vastly outnumbered the defenders, yet the castle held. The waterlogged terrain frustrated conventional siege tactics, and the defenders used the same wetlands that made the castle hard to approach. The siege became a celebrated episode in the castle’s history, and it is the primary reason Oshi Castle is remembered today.

During the Edo period, Oshi Castle became the centre of the Oshi Domain, a fief assessed at 100,000 koku. The domain was administered from the castle, which functioned as the political and military headquarters of the region. The castle remained in use through the Tokugawa shogunate’s rule.

The castle’s active life ended in 1873. Following the Meiji Restoration, the new government abolished most feudal fortifications, and Oshi Castle was dismantled. Its structures were removed, and the site lost its military function.

What Stands There Today

The original castle buildings no longer exist. The site now hosts the Gyōda City Museum, which serves as the castle’s modern successor. The museum preserves and presents the history of the castle and the surrounding area.

The castle’s earthworks and moats remain visible in parts, giving visitors a sense of the original defensive layout. The wetlands that once made the castle so difficult to attack have been partially preserved, and they still shape the character of the site. The location is marked by its coordinates at 36.1371000, 139.4530000, in the city of Gyōda.

The Name in Japanese

The castle’s name in Japanese is 忍城. Its kana reading is おしじょう, and its romaji is Oshi-jō.

The name breaks down into two kanji:

• 忍 (oshi) — the character means "to endure" or "to bear". In this word it is read as oshi, though the character is more commonly associated with the reading shinobu. It is the same character used in the name of the famous ninja village Iga, where it appears in the compound 忍び (shinobi).
• 城 (jō) — the character means "castle". It is read as jō in this word, a reading that appears in countless Japanese castle names, such as Osaka Castle (大坂城, Ōsaka-jō) and Himeji Castle (姫路城, Himeji-jō).

The full name, Oshi-jō, therefore reads literally as "Enduring Castle" or "Castle of Endurance". The nickname Kama-jō (亀城) uses the character 亀, meaning "turtle", and 城, meaning "castle", giving "Turtle Castle". The other nickname, Oshi-no-uki-jō (忍の浮き城), combines 忍 (endure), の (a possessive particle), 浮き (floating), and 城 (castle), producing "the Floating Castle of Oshi".

Visiting and Context

Oshi Castle is located in Gyōda-shi, in Saitama Prefecture. The site is accessible as part of a visit to the Gyōda City Museum, which stands on the castle grounds. The museum offers the clearest entry point for understanding the castle’s history, including the Siege of Oshi.

The castle’s place in the 関東七名城, the seven great castles of the Kantō region, gives it a standing beyond its local importance. Visitors who know the other six castles will find Oshi distinct for its wetland setting and its unusual defensive story. The nickname "Floating Castle" is not mere poetry; the castle’s survival in the Siege of Oshi depended on the same water and marsh that earned it that name.

The castle’s history spans nearly four centuries, from its construction in 1479 to its dismantling in 1873. That span covers the Muromachi period, the Sengoku wars, and the long peace of the Edo period. The site today is a quiet place, but the earthworks and the museum keep the memory of the siege and the domain alive.