The Birth of a Mountain Castle
The construction of Koiwakate Castle is traditionally dated to 1522, when it was built by Nishina Morikuni. The castle stood in the former Furumaya area of Hotaka-Ariake and was designed primarily as a defensive stronghold.
At the time, the Nishina clan repeatedly came into conflict with the Ogasawara clan, the shugo, or military governor, of Shinano Province. Although the Nishina eventually entered into a subordinate relationship with the Ogasawara, political tensions remained.
Koiwakate Castle was therefore built with defense in mind. Its position on a mountain and its network of earthworks allowed a relatively small force to defend the stronghold against a larger attacking army.
The Siege by Takeda Shingen
The castle’s most dramatic chapter came during the expansion of the Takeda clan. During the Tenbun era, Takeda Harunobu, later known as Takeda Shingen, began expanding from Kai Province into Shinano.
In August 1552, a Takeda army of approximately 3,000 soldiers surrounded Koiwakate Castle. The defenders, led by the castle lord Koiwa Morichika, numbered around 500 men and resisted the attacking force.
According to tradition, the siege continued for approximately three months. In the end, Morichika took his own life and the castle fell. The battle marked a major turning point for the Nishina clan and its influence in the Azumino region.
In 1561, the main Nishina line also came to an end, bringing the period of Nishina dominance in Azumino to a close.
From Takeda Rule to the Castle’s Abandonment
After the fall of Koiwakate Castle, the Furumaya clan became the local ruling family. They submitted to the Takeda clan and came under the authority of Nishina Morinobu, lord of Mori Castle.
The political landscape changed dramatically in 1582, when the Takeda clan was destroyed. Azumino was subsequently assigned to Ichikawa Nobufusa, a retainer of Uesugi Kagekatsu. Later, Ogasawara Sadayoshi recovered his former territory, and Furumaya Morikatsu pledged allegiance to him.
However, in 1583, Morikatsu was suspected of secretly communicating with the Uesugi side and was executed by the Ogasawara clan, according to historical tradition. Following this incident, Koiwakate Castle is believed to have lost its role and was abandoned along with nearby Furumaya Castle.
Exploring Koiwakate Castle Ruins Park
Today, the former castle site has been developed as Koiwakate Castle Ruins Park. Reconstructed features, including a wooden gate resembling a traditional castle turret gate and a lookout structure built on stonework, help visitors imagine the appearance and strategic character of the original fortress.
The ruins extend across both the mountaintop and lower slopes. Rather than relying on reconstructed buildings alone, the surviving earthworks and stone structures provide important evidence of how a Japanese mountain castle was actually designed.
Defensive Structures on the Mountain
More than 50 baileys, or kuruwa, of various sizes are believed to have been arranged across the upper part of the castle. Flattened areas spread particularly along the northern ridge, creating a series of defensive positions.
Several tatebori, or vertical moats cut into the hillside, were placed in succession along the western ridge. These earthworks made it difficult for an attacking force to approach the castle directly and demonstrate the sophisticated defensive techniques used at medieval Japanese mountain fortresses.
The arrangement is especially interesting because such castles were often designed to make effective use of limited manpower. Natural slopes, ridges, trenches, and elevated positions were incorporated into a single defensive system.
Remains at the Foot of the Mountain
The lower section of the castle contains remains of earthen ramparts, stone walls, and entrances known as koguchi. Some of the stone walls use a traditional technique called nozurazumi, in which naturally shaped stones are stacked with relatively little processing.
The rough, powerful appearance of these stone walls contrasts with the carefully arranged earthworks surrounding them. Together, they reveal how the castle’s defenses were adapted to the natural terrain.
A large dry moat once surrounded the lower castle area. It measured approximately 15 meters wide and 7 meters deep, with angled sections designed to allow defenders to attack enemies from the side. These features illustrate the strategic thinking behind the fortress.
A View Across the Azumino Countryside
The reconstructed lookout provides one of the most enjoyable features of the present-day park. From the elevated position, visitors can look across the peaceful Azumino countryside and appreciate why the location was valuable as a military stronghold.
Rice fields, wooded hills, and the surrounding mountains create a striking contrast with the site’s turbulent history. Standing at the former castle’s highest point makes it easier to understand how medieval defenders could monitor movement across the surrounding landscape.
The park also contains a memorial monument for the soldiers who died in the battles with the Takeda army. It gives the site a quiet and reflective atmosphere, reminding visitors that these beautiful hills were once the setting for violent conflict.
History in a Peaceful Natural Setting
One of the most appealing aspects of Koiwakate Castle Ruins is the contrast between its past and present. The castle once stood at the center of a struggle for control of Shinano, yet the surrounding landscape is now characterized by forests, fields, and the peaceful scenery of Azumino.
Weathered stone walls, old earthworks, and pine trees create an atmosphere in which it is easy to imagine the castle as it may have appeared centuries ago. Unlike a completely reconstructed castle, the site invites visitors to read the landscape itself and discover how natural terrain was transformed into a defensive fortress.
Visiting Koiwakate Castle Ruins
The ruins can be reached from JR Hotaka Station by taking the Azumino sightseeing bus and getting off at the Koiwatake Villa Entrance bus stop. From there, it is approximately a 20-minute walk to the castle ruins park.
By car, the journey from the Nagano Expressway Azumino Interchange takes approximately 30 minutes. Free parking is available, making the site convenient for visitors exploring Azumino by car.
A Window into the Warring States Era
Koiwakate Castle Ruins offers a fascinating glimpse into the turbulent history of medieval Japan. The story of the castle encompasses the Nishina clan, the expansion of Takeda Shingen, the struggles among powerful regional warlords, and the political changes that followed the destruction of the Takeda clan.
Today, there are no armies surrounding the mountain and no battles taking place among the ridges. Instead, visitors can walk quietly through the ruins, examine the surviving earthworks and stone walls, and look out over the peaceful Azumino landscape.
For travelers interested in Japanese castles, samurai history, archaeology, and mountain scenery, Koiwakate Castle Ruins Park provides a rewarding opportunity to experience a lesser-known chapter of the Warring States period while enjoying the natural beauty of Azumino.