Shirai Castle was a castle that once stood in Shirai, Shibukawa City, Gunma Prefecture. From the Muromachi period through the early Edo period, it served as an important strategic base in the region. Today, the site is preserved as the Shirai Castle Ruins and is designated as a Shibukawa City Historic Site.
The castle was built on a river terrace near the confluence of the Tone and Agatsuma rivers. The steep cliffs and other natural features surrounding the site were skillfully incorporated into its defensive design, making it a highly defensible castle.
The name “Shirai Castle” is sometimes pronounced “Shirai-jo,” but locally it has traditionally been known as “Shiroi-jo.” Although none of the original castle buildings remain, features such as stone walls, earthen ramparts, and moats can still be seen around the main bailey, offering a glimpse into the appearance of a castle during Japan’s Warring States period.
Shirai Castle was built on a tongue-shaped plateau projecting near the confluence of the Tone and Agatsuma rivers. The steep cliffs facing the rivers served as natural defensive barriers, making the location highly suitable for protecting the castle from attack.
The castle grounds covered a vast area of approximately 800 meters from east to west and 1,200 meters from north to south. The main bailey was located at the center, surrounded by various fortified sections, including the second and third baileys, the Northern Bailey, and the main outer enclosure.
Additional fortified areas, including the Southern Bailey and New Bailey, were located on the southern side. The overall castle complex is believed to have had a roughly triangular shape with a pointed southern end.
The western side was protected by steep cliffs overlooking the Agatsuma River, while Shirai-juku, which developed as the castle town, spread to the southeast. Because of this strategic location, Shirai Castle played an important role in controlling transportation routes connecting Echigo and the Kanto region.
The exact date when Shirai Castle was built is unknown, but it is generally believed to have been constructed around the middle of the 15th century, during the time of Nagao Kagenaka, a retainer of the Yamanouchi Uesugi clan.
The Nagao family based in Shirai became known as the Shirai Nagao clan. For generations, they used Shirai Castle as their base and expanded their influence throughout the surrounding region.
During the Warring States period, competition for power intensified throughout the Kanto region, and Shirai Castle became involved in these conflicts. Its location along an important transportation route made it a strategically significant castle for controlling the surrounding area.
One of the most important figures in the history of Shirai Castle was Nagao Kageharu of the Shirai Nagao clan. Kageharu rebelled against the Yamanouchi Uesugi clan, and Shirai Castle became an important base during the conflict.
As wars continued, control of the castle changed hands several times, making Shirai Castle one of the sites that reflected the intense power struggles taking place throughout the Kanto region.
Later, when the powerful warlord Takeda Shingen of Kai expanded his campaigns into western Kozuke, the castle was reportedly captured in 1567 by forces led by Takeda retainers including Sanada Yukitsuna, also known as Sanada Yukitaka, and Sanada Nobutsuna.
The Shirai Nagao clan later regained control of the castle, and Shirai Castle continued to be a site of conflict on numerous occasions throughout the Warring States period.
A major turning point in the history of Shirai Castle came with Toyotomi Hideyoshi’s campaign against Odawara.
In 1590, large armies from across Japan advanced into the Kanto region as Hideyoshi sought to defeat the Hojo clan. Many castles were attacked and captured during the campaign. Shirai Castle was also attacked, and forces led by Maeda Toshiie and Uesugi Kagekatsu occupied the Northern Bailey. The castle was subsequently surrendered on May 15 of the same year.
This brought the long period of dominance by the Shirai Nagao clan in the region to an end.
After Hideyoshi’s campaign against Odawara, Tokugawa Ieyasu entered the Kanto region, and Shirai Castle was used as the base of the Shirai Domain.
Various feudal lords, including the Honda, Matsudaira, Ii, and Nishio families, subsequently became lords of the castle. It continued to serve as a castle until the early Edo period.
However, when the Shirai Domain was abolished in 1623, Shirai Castle was also abandoned. The castle buildings were dismantled, and the once-extensive fortress gradually disappeared from the landscape.
Today, much of the former castle grounds has become farmland and residential areas, but important remains can still be found around the main bailey.
Near the entrance to the main bailey, visitors can see the remains of the koguchi, a strategically designed castle entrance, along with sections of stone wall. These features provide an opportunity to observe how castle entrances were designed to hinder enemy attacks and imagine the defensive strategies used at the time.
Remains of earthen ramparts and dry moats can also be found around the site. These features offer a sense of the enormous scale of the former castle, even though the area is now a quiet landscape of fields and residential surroundings.
In 2004, the area around the main bailey was designated as a Shibukawa City Historic Site, and preservation and maintenance efforts have continued.
Near Shirai Castle Ruins is Shirai-juku, the historic settlement that developed as the castle town. Traditional streetscapes remain in the area, and visiting both locations provides a deeper understanding of the history of Shirai.
After exploring the castle ruins, visitors can walk through Shirai-juku and experience the relationship between the castle of the Warring States period and the town that developed around it.
Shirai Castle Ruins are located in Shirai, Shibukawa City, Gunma Prefecture. The site is approximately 15 minutes by car from JR Shibukawa Station on the Joetsu Line.
The surrounding area also features natural scenery along the Tone and Agatsuma rivers, as well as rural landscapes. A visit therefore offers opportunities to enjoy both the area’s history and its natural environment.
Shirai Castle Ruins preserve the history of a castle that served as the stronghold of the Shirai Nagao clan and played a role in the power struggles of the Warring States period and the campaign against Odawara.
Although the original buildings have disappeared, remains such as the stone walls, koguchi entrance, earthen ramparts, and dry moats around the main bailey allow visitors to imagine the scale and defensive structure of the former castle.
Combining a visit to the ruins with a walk through Shirai-juku makes it possible to experience the history of both the castle and its surrounding castle town. It is an interesting historic site for visitors who want to discover the history of Shibukawa and Japan’s Warring States-era castles.