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Castle Koizumi Castle

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This article is in the series Walking the Japanese Castles
Koizumi Castle (小泉城 Koizumi-jō) is a former flatland castle located in Oizumi (大泉町), Ora-gun, in southeastern Gumma Prefecture. The castle was built in 1489, the first year of the Entoku era (延徳, 1489-1492). Today, only the ruins of the castle (all of the inner moat, part of the outer moat and earthworks) remain, and the area has been developed into Shiro-no-uchi Park (城之内公園).


The castle was built by Ashikaga Shigeuji's son Mochimitsu (later known as Tomioka Naomitsu, 冨岡直光), who lost in the Siege of Yūki (結城合戦) in 1440 and was later given the territory of Ora-gun. It was around this time that it came to be known as Koizumi Castle. The Tomioka clan played an active role under the Koga kubō (古河公方), Uesugi Kenshin (上杉謙信, 1530-1578), and the Late Hōjō. Later, the Koizumi clan formed a new family branch independent of the Tomioka. However, in 1590, when Toyotomi Hideyoshi invaded the Kantō region, the Koizumi were attacked by Asano Nagamasa (浅野長政, 1546-1611), Hideyoshi's brother-in-law; the castle fell and was abandoned.

Between the remaining sotobori (外堀, outer moat of a castle) and uchibori (内堀, inner moat), there are more than a dozen oval stone pillars engraved with Kōshin (庚申) symbols believed to date back to the Edo period when the Kōshin faith, a folk belief with Taoist origins, influenced by Shintō and Buddhism, was flourishing.

The castle consists of a honmaru (本丸, inner enclosure), a ninomaru (二ノ丸, second enclosure) and a sannomaru (三ノ丸, third enclosure). The moats have been renovated and the earthworks still are in good shape; the moat in the photo below is situated on the castle's north side.


Based on the Aerial Photograph Searching Service of the Geospatial Information Authority of Japan. The location of the enclosures is estimated.


Local guide board


Earthworks on the west side of the sannomaru.


The view of the sannomaru from the honmaru.


There is a school on the south side of the sannomaru.


This is the moat surrounding the honmaru.


The honmaru is surrounded by earthwork as well. Incidentally, Oizumi Town is home to a large community of foreign workers. I saw a Japanese-Brazilian family playing football in the park.


Date of visit: 24 November 2013


  • Address: 2-21-11 Shironouchi, Oizumi, Ora District, Gunma 370-0518
  • Access: an 8-minute walk from Tōbu Koizumi Line (東武小泉線) Koizumi-cho Station
    9 kilometres from Kita-Kanto Expressway Ota-Kiryu Interchange via National Route 122
  • Parking: in the park (see the 'P' in the map above)
  • Other sights: Ashikaga Residence
Next article in the series 'Walking the Japanese Castles': Kanayama Castle
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Hiroto Uehara
Hiroto is an ordinary Japanese office worker, but his true mission is searching for castles on the weekend.


My wife and I visited the castle in the same year. I remember hearing a lot of Portuguese in the castle park, too. :)

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Hiroto Uehara
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〒370-0518 群馬県邑楽郡大泉町城之内2丁目21−11

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