Rising above Higashiagatsuma Town in Gunma Prefecture, Mount Iwabitsu is home to the historic ruins of Iwabitsu Castle. Built on the mountainside overlooking the northern bank of the Agatsuma River, this formidable mountain fortress played an important role during Japan’s turbulent Sengoku period.
The castle became a strategic battleground between the Saitō clan, who were associated with Uesugi Kenshin, and the Sanada clan, who served the Takeda clan. It eventually came under the control of Sanada Nobuyuki and was abandoned in 1614.
Today, the castle ruins are preserved as a National Historic Site and attract history enthusiasts, hikers, and visitors interested in Japan’s samurai heritage.
The exact date when Iwabitsu Castle was built remains uncertain. According to local tradition, it was constructed by Azuma Tarō Sukesuke, who ruled the area during the Kamakura period. However, there is no definitive historical evidence to confirm this account.
The Azuma clan later became caught up in the conflicts of the Northern and Southern Courts period. The political situation surrounding the castle became increasingly unstable, and the clan leader Yukimori was killed in battle.
Yukimori’s son, Chiōmaru, was placed under the protection of the Akima Saitō clan. He later received a name character from the Yamauchi branch of the Uesugi clan and became known as Saitō Noriyuki. Iwabitsu Castle gradually developed into an important stronghold of the Saitō clan as the region moved toward the Sengoku period.
During the Sengoku period, Saitō Noritsugu is said to have defeated his lord and seized Iwabitsu Castle. His son, Saitō Norihiro, subsequently used the castle as his base while ruling much of Agatsuma District.
In 1560, Nagao Kagetora, who later became known as Uesugi Kenshin, is said to have captured the castle. However, the Saitō clan eventually regained control.
The situation changed dramatically when the Sanada and Takeda clans became involved in conflicts among the local lords. In 1563, an offensive led by Sanada Yukitaka, combined with internal cooperation from within the castle, resulted in the fall of Iwabitsu Castle.
The Saitō clan fled to Echigo, and its influence in the region was eventually eliminated.
For the Sanada clan, Iwabitsu Castle was a strategically important stronghold connecting Numata Castle and Nagurumi Castle in northern Kōzuke Province with Ueda Castle in Shinano Province.
The Sanada extensively remodeled the fortress, creating an unusual defensive layout in which moats extended outward from the central enclosures. Additional outer defenses, forts, and guard positions were established throughout the surrounding area.
In 1582, when Takeda Katsuyori came under severe pressure from the combined forces of Oda Nobunaga and Tokugawa Ieyasu, Sanada Masayuki reportedly proposed that Katsuyori retreat to Iwabitsu Castle.
It has sometimes been suggested that history might have taken a different course had Katsuyori entered the castle, but the proposal was not realized, and the Takeda clan was ultimately destroyed.
Masayuki later entrusted the defense of Iwabitsu Castle to his eldest son, Sanada Nobuyuki, who was also known as Nobuyuki. In 1614, however, the castle was dismantled, reportedly out of consideration for the Tokugawa shogunate. A fortified residence was subsequently established in Haramachi, and the center of local administration was moved there.
Iwabitsu Castle was built along a ridge on the northeastern side of Mount Iwabitsu. Its natural surroundings provided powerful defensive advantages.
Mount Iwabitsu rises to the west, the steep slopes descending toward the Agatsuma River protect the southern side, and rocky terrain surrounds the north. The landscape itself therefore functioned as a natural defensive wall, making Iwabitsu a classic example of a mountain fortress protected by natural terrain.
The main enclosure, second enclosure, and central enclosure were arranged along the mountain ridge, while an outer fortification known as Tengumaru was positioned to the northeast.
The wider defensive network also included Yanagisawa Castle, also known as Kannonyama Castle. The castle’s extensive system of fortifications is sometimes regarded as an early example of the defensive ideas later associated with other Sanada strongholds, including Ueda Castle and the Sanada Maru at Osaka Castle.
Iwabitsu Castle was designated a town historic site in 1972 and selected as No. 117 on the Continued 100 Fine Castles of Japan list in 2017. It was designated a National Historic Site in 2019.
Recent archaeological investigations have produced intriguing discoveries. A fragment of a crucible used for metalworking was found at the eastern end of the main enclosure. Scientific analysis detected particles of gold in the material.
This discovery suggests that gold processing may have taken place at the castle during the period of the Sanada or Takeda clans. The findings indicate that Iwabitsu Castle may have served not only as a military stronghold but also as a site with important economic and production functions.
Standing at 802.6 meters, Mount Iwabitsu is recognized as one of the 100 Famous Mountains of Gunma. Together with Agatsuma Gorge, it is also one of the scenic highlights of the Agatsuma Eight Views.
The southern face features dramatic cliffs rising approximately 200 meters, creating the rugged profile that makes the mountain so striking.
The mountain was formed by volcanic activity approximately six million years ago. Over countless years, wind, rain, and river erosion shaped the rock formations into the distinctive landscape visible today.
Chains and ladders are installed along sections of the hiking trails, allowing visitors to experience an appropriate degree of adventure as they make their way toward the summit. From the top, hikers can enjoy panoramic views of the surrounding mountains of Gunma, and on clear days, Mount Fuji may also be visible in the distance.
Every year on November 3, a local autumn foliage festival is held, featuring seasonal mountain scenery as well as regional dishes and wild mountain vegetables. The event attracts many visitors.
Iwabitsu became widely known to a new generation of visitors after Mount Iwabitsu appeared during the climactic moments of the opening sequence of the 2016 NHK historical drama series “Sanada Maru.”
The area is closely associated with the early history of famous Sanada figures such as Sanada Masayuki and his son Sanada Nobushige, better known as Yukimura. The combination of dramatic mountain scenery and Sengoku history has made Iwabitsu a popular destination for visitors interested in Japan’s samurai heritage.
At the foot of the mountain lies the ruins of Senryūin, traditionally believed to be the site of a residence built by Sanada Masayuki in preparation for receiving Takeda Katsuyori. It is another historical site worth visiting together with the castle ruins.
Iwabitsu Castle and Mount Iwabitsu bring together history, natural scenery, and dramatic mountain landscapes, making them among the highlights of Higashiagatsuma Town.
Walking among the remains of the mountain fortress offers an opportunity to imagine the warriors and political struggles of the Sengoku period, while the panoramic views from Mount Iwabitsu provide a completely different perspective on the surrounding region.
The mountain changes its character with the seasons. Fresh green foliage in spring, deep greenery in summer, colorful autumn leaves, and the crisp air of winter each create a distinctive atmosphere.
For visitors interested in Japan’s samurai history as well as those who enjoy hiking and spectacular scenery, a journey around Iwabitsu offers a memorable way to experience the history and natural beauty of Higashiagatsuma.