Koizumi Castle, also known as Tomioka Castle, was a castle built during Japan’s Warring States period in what is now Oizumi Town, Ora District, Gunma Prefecture. The castle is traditionally believed to have been constructed in 1489.
Today, the former castle grounds have been preserved as Shironouchi Park. Parts of the original castle layout remain, including several kuruwa fortified compounds, sections of the inner and outer moats, and earthen ramparts. The site is designated as a Historic Site by Oizumi Town.
The castle ruins and the reconstructed Shironouchi Kofun are collectively designated as a cultural property under the name “Koizumi Castle Ruins (Tomioka Castle Ruins), including the reconstructed Shironouchi Kofun.” Together, they represent an important part of Oizumi’s historical and cultural heritage.
Koizumi Castle is traditionally said to have been built by Mochimitsu, the son of Yuki Mochitomo, who was defeated in the Yuki War. After receiving land in Ora District from Ashikaga Shigeuji, Mochimitsu, later known as Tomioka Chikara-no-suke Naomitsu, is believed to have established the castle in 1489.
The castle was originally known as Tomioka Castle, but was later renamed Koizumi Castle after the local place name.
The Tomioka clan subsequently became active under the influence of the Koga Kubo and governed the surrounding area during the Warring States period while maintaining relationships with powerful regional leaders such as Uesugi Kenshin and the Later Hojo clan.
Based at Koizumi Castle, the Tomioka family expanded its influence and is believed to have ruled the area as castle lords for approximately 100 years.
The situation changed dramatically in 1590, during Toyotomi Hideyoshi’s campaign against the Hojo stronghold at Odawara.
Because Koizumi Castle was aligned with the Later Hojo clan, it was attacked by forces on Hideyoshi’s side, including the army of Asano Nagamasa. The castle eventually fell, and after the campaign it was abandoned, bringing its history as a Warring States stronghold to an end.
Today, much of the inner moat, sections of the outer moat, and several earthen ramparts remain. These features provide valuable evidence of the castle’s original defensive structure.
Unlike mountain castles, Koizumi Castle was built as a hirajiro, or flatland castle. Because Oizumi has no high mountains, the castle was constructed on a relatively elevated area of land at approximately 33 meters above sea level.
The castle followed a design known as kaku-kakuri-shiki, in which the central fortified compound was surrounded by moats and earthen ramparts. This was a typical defensive arrangement for a flatland castle.
During the late Warring States period, the castle was renovated under the control of the Later Hojo clan. Many of the remains visible today are thought to date from this period.
It is traditionally said that two- to three-story corner towers once stood at the northeastern and southwestern corners of the main compound. Slightly elevated areas that remain around the site are believed to mark the locations of these towers.
One distinctive feature of the castle ruins is the large number of oval-shaped stone pillars inscribed with the characters “Koshin”, found between the inner and outer moats.
These stones are associated with Koshin worship, a popular folk religious tradition during the Edo period. Local people are believed to have erected such stones as prayers for good health, protection from illness, and longevity.
More than several dozen of these stone pillars stand to the west of the Chureito Memorial Tower, creating a distinctive landscape that preserves not only the history of the castle but also the religious traditions of the local community.
The grounds of Shironouchi Park are also home to the reconstructed Shironouchi Kofun, an ancient burial mound dating from Japan’s Kofun period.
The burial mound was reconstructed as part of efforts to preserve the area’s ancient archaeological heritage. It is now maintained alongside the castle ruins as part of the historic park.
As a result, Koizumi Castle Ruins preserve more than the remains of a Warring States-period castle. The site also incorporates an ancient burial mound, allowing visitors to experience layers of local history spanning many centuries.
Today, the former Koizumi Castle site has been developed as Shironouchi Park, a popular place for residents and visitors to relax.
The park features walking paths and open spaces where visitors can enjoy a leisurely stroll while exploring the historic landscape and surrounding greenery.
In spring, approximately 300 cherry trees burst into bloom, making the park one of Oizumi’s popular cherry-blossom viewing spots. The combination of full-bloom cherry trees and the historic castle landscape creates one of the town’s characteristic spring scenes.
The park also features a mini zoo and recreational facilities, making it an enjoyable destination for families as well as history enthusiasts.
With history, nature, and everyday community life coming together in one place, Shironouchi Park continues to be a beloved local destination.
Koizumi Castle Ruins are a valuable historic site where visitors can experience the remains of a Warring States-period castle at close range. The site is essential for understanding the history of Oizumi Town and the region.
Walking along the surviving moats and earthen ramparts provides an opportunity to imagine how the flatland castle was structured and how its defensive system once functioned.
Because the site is now surrounded by greenery and maintained as a public park, it is easy to combine historical exploration with a relaxing walk. When visiting Oizumi, take some time to explore Shironouchi Park and discover the history and local culture preserved in the former grounds of Koizumi Castle.