Nagurumi Castle is a historic castle site located in Shimozu, Minakami, Gunma Prefecture. Built on a steep cliff overlooking the upper reaches of the Tone River, this mountain castle made clever use of the surrounding natural terrain to create a formidable defensive position.
Today, the site is maintained as the Nagurumi Castle Ruins. It was designated a Gunma Prefectural Historic Site in 1949, and in 2017 it was selected as No. 115 on Japan’s “100 Fine Castles Continued”, a list recognizing historically significant castles throughout Japan. The site attracts history enthusiasts and visitors interested in Japan’s Sengoku period.
Nagurumi Castle stands near the confluence of the Tone River and Akaya River, on a naturally defensible position surrounded by steep cliffs on three sides. This rugged terrain provided the castle with strong natural defenses and made it an important military position.
Across the Tone River stood Myotokuji Castle, creating a strategic situation in which the two fortifications faced each other from opposite sides of the river.
The castle was built in a linear compound-style layout, with the main enclosure, second enclosure, and third enclosure arranged along a line. Defensive features such as earthen walls, moats, and castle entrances were incorporated into the design.
Many of these remains have survived in relatively good condition, allowing visitors to gain a sense of how a mountain castle functioned during Japan’s Sengoku period, an era of intense warfare and political upheaval from the 15th to 16th centuries.
The origins of Nagurumi Castle can be traced back to the Muromachi period. According to local tradition, in 1492, the Nagurumi clan, a branch of the powerful Numata family, established a residence known as Nagurumi Yakata here.
At that time, the site was not yet a full-scale mountain castle like the one that existed during the Sengoku period. It is believed to have functioned primarily as a fortified residence and local administrative base.
During the latter part of the Sengoku period, the Takeda clan of Kai Province expanded its influence into the region of present-day Gunma. In 1578, under orders from Takeda Katsuyori, the samurai commander Masayuki Sanada advanced into the Agatsuma and Tone areas and captured Nagurumi Yakata.
Sanada then built a new castle on the site, using it as a forward base for an attack on Numata Castle.
From Nagurumi Castle, Sanada successfully used political maneuvering and persuasion to gain control of Numata Castle. The Sanada clan subsequently expanded its influence throughout the region, and Nagurumi Castle became an important base supporting the clan’s expansion into northern Kanto.
After the Takeda clan fell in 1582, a new struggle for power unfolded across the Kanto region. The Sanada clan maintained its independence and became involved in a prolonged conflict with the powerful Late Hōjō clan over control of the Numata and Agatsuma areas.
Nagurumi Castle served as a satellite fortress of Numata Castle and played an important role in defending the territory against Hōjō forces.
Its steep terrain and strong defensive position made the castle difficult to attack, and historical accounts state that it successfully resisted repeated attacks by the Hōjō forces.
In 1589, as Toyotomi Hideyoshi was advancing toward the unification of Japan, he issued an order known as the “Sōbujirei”, which prohibited feudal lords from engaging in unauthorized warfare against one another.
Hideyoshi also settled the dispute over the Numata territory. The Tone River was established as the boundary, with the territory to the east assigned to the Hōjō clan and the territory to the west assigned to the Sanada clan. Nagurumi Castle was therefore officially recognized as Sanada territory.
However, in November of the same year, Kuninori Inomata, a Hōjō retainer and commander of Numata Castle, used a deception to seize Nagurumi Castle.
Shigenori Suzuki, the commander of Nagurumi Castle, was lured away from the castle by a forged letter. While he was absent, Hōjō forces occupied the castle. According to tradition, Suzuki took responsibility for the loss and later committed suicide at Shōkakuji Temple.
The seizure of Nagurumi Castle angered Toyotomi Hideyoshi, who regarded the incident as a violation of his order prohibiting unauthorized warfare. The incident became one of the direct triggers for Hideyoshi’s decision to launch the Odawara Campaign.
In 1590, a large Toyotomi army marched into the Kanto region and surrounded Odawara Castle, the principal stronghold of the Hōjō clan.
The campaign resulted in the downfall of the Hōjō clan and brought Hideyoshi’s long campaign to unify Japan to completion. In this way, the Nagurumi Castle Incident became an important turning point in Japanese history, connecting a relatively small local conflict with events that reshaped the political landscape of the entire country.
Following the Odawara Campaign, the Sanada clan was allowed to retain control of the Numata territory. However, once the period of major warfare came to an end, Nagurumi Castle lost its military importance and was eventually abandoned.
The castle is believed to have been used for only around ten years in its full-scale form.
Because the site underwent relatively little major alteration after its abandonment, many of its original features remain visible today. The ruins provide valuable evidence of the structure and defensive techniques of a Sengoku-era mountain castle.
The ruins preserve a distinctive linear arrangement of fortified compounds, extending from the outer defensive area through the third enclosure and second enclosure to the main enclosure and Sasaguru enclosure.
Archaeological investigations have uncovered numerous remains, including earthen walls, a crescent-shaped moat, castle entrances, pathways, gate foundation stones, and post-hole remains of wooden buildings.
The castle was designed to take maximum advantage of the natural landscape. From the steep cliffs overlooking the Tone River basin, visitors can enjoy views across the surrounding area.
Walking through the ruins while imagining the samurai commanders who once stood here planning their strategies is one of the most rewarding ways to experience Nagurumi Castle.
At the center of the main enclosure stands the “Monument to the Nagurumi Castle Ruins”, erected in 1924 by the Nagurumi Castle Preservation Association.
The inscription was written by Sohō Tokutomi, a prominent historian and writer of the Meiji and Taisho eras. The monument serves as an important reminder of the site’s historical significance and its preservation for future generations.
At the entrance to the castle grounds is the Nagurumi Castle Ruins Information Center, where visitors can learn about the castle’s history and the major events of the Sengoku period through exhibits and historical materials.
The former site of Nagurumi Yakata, known as the Hannya Enclosure, has been developed as a parking area, making the historic site convenient for visitors arriving by car.
Address: 3437 Shimozu, Minakami, Tone District, Gunma Prefecture
By car: Approximately 6 minutes from the Tsukiyono Interchange on the Kan-Etsu Expressway.
By train: Approximately 7 minutes by taxi from Jōmō-Kōgen Station on the Joetsu Shinkansen or Gokan Station on the JR Joetsu Line.
Nagurumi Castle is an important historic site for understanding the turbulent Sengoku period of Japanese history. It was a stage for the activities of Masayuki Sanada and, more significantly, the setting of the Nagurumi Castle Incident, an event that contributed to Toyotomi Hideyoshi’s decision to launch the campaign against the Hōjō clan.
Today, visitors can walk among the preserved castle remains and experience the layout of a Sengoku-era mountain fortress while surrounded by the natural scenery of Minakami.
For travelers exploring the Minakami area, a visit to Nagurumi Castle offers a fascinating opportunity to stand on a historic site where a local struggle once became connected to one of the major turning points in the unification of Japan.