Guide
Iwabi-jo Castle is a massive mountain fortress located on Mt. Iwate (802m) in Higashiaguma Town, Gunma Prefecture. It was historically recognized as one of the "Three Great Fortresses of the Takeda Clan," alongside Iwado Castle in Kai and Kunō Castle in Suruga. The site is characterized by a complex structure that utilizes the natural terrain, featuring deep moats, defensive embankments (kurawa), and sheer cliffs that served as natural defenses. Now designated as a national historical site, visitors can experience the profound weight of history set against a stunning natural landscape.
Tracing back to the Nanboku-cho period, legend suggests the castle was originally built by the Agatsuma clan. During the Sengoku (Warring States) period, the region was a site of intense conflict between the Saito clan (allied with Uesugi Kenshin) and the Sanada clan (allied with Takeda Shingen). The Sanada clan, in particular, heavily fortified and remodeled Iwabi-jo to serve as a strategic stronghold connecting Numata Castle and Ueda Castle. Under the leadership of Sanada Masayuki, the castle functioned as a vital base for the Takeda clan's counter-offensives. Although the Sanada family controlled the site, the castle was eventually decommissioned in 1614 due to the orders of the Shogunate. Recent archaeological findings, including metal processing artifacts and gold particles near the Honmaru (inner sanctum), highlight the advanced technology and economic activity of the era.
The most striking feature of the ruins is the defensive architecture integrated with the natural topography. As you ascend toward the Honmaru, you will encounter vertical grooves (tatebori) stretching over 100 meters, complex defensive embankments, and deep moats. At the Honmaru, there is a small structure called "Azumaya" where visitors can sign a mountain climber's notebook. Another fascinating sight is a massive round rock near the Honmaru, which serves as a testament to the incredible engineering and transport capabilities of the time. Additionally, the outpost known as "Tengumaru" to the northeast offers a clear view of the castle's formidable defensive strength.
As a mountain castle, Iwabi-jo offers beautiful seasonal changes. Spring and autumn are particularly delightful, as the changing colors of the surrounding mountains complement the historical atmosphere. While the hiking trails are maintained, please be aware that the high altitude means weather and temperatures can change rapidly. Visiting during daylight hours is recommended to clearly observe the structural details of the moats and fortifications.
Experience an authentic trek through ancient military history. It takes approximately 30 minutes to reach the Honmaru via a maintained trail. Walking through the vertical grooves and defensive embankments allows you to physically experience the military tactics of the Sengoku period. Don't forget to leave a message in the mountain climber's notebook at the Honmaru to commemorate your visit.
The area surrounding Mt. Iwate is rich in nature. Higashiaguma Town and its surroundings are steeped in history, offering many opportunities to explore the legacy of the Sanada clan. You can also enjoy the scenic beauty of the Agatsuma River nearby, making it a perfect spot for a day of nature-filled exploration after your historical hike.
Because Iwabi-jo is a high-altitude mountain castle, the path to the summit includes steep stairs and inclines. Sturdy hiking boots or sneakers and comfortable athletic clothing are essential. Since you will be in a mountainous area, please bring a rain jacket and plenty of water. As this is a protected historical site, please respect the environment by carrying all trash back with you and avoiding any damage to the ruins. Toilets are available near the Honmaru area.