A Mountain Fortress at an Elevation of 120 Meters
A Strategic View over the Masuda Plain and the Sea of Japan
Nanao Castle was built on Mount Nanao at an elevation of approximately 120 meters. The former main bailey, located near the summit at around 118 meters above sea level, commands wide views over the Masuda Plain and the Sea of Japan.
This elevated position provided excellent natural defenses while also serving as a strategic observation point from which the Masuda clan could oversee its territory.
The castle extended for approximately 600 meters from north to south along a mountain ridge. More than 40 fortified enclosures, known as kuruwa, were arranged along the Y-shaped terrain.
Remains of dry moats, earthen ramparts, wells, and other defensive structures can still be found throughout the site. One particularly notable feature is the stone-lined well known as Umatsurui, a rare surviving example of a well associated with a Japanese mountain castle.
A History Shaped by Medieval Warfare
The Traditional Foundation Date
The exact date when Nanao Castle was first constructed remains uncertain. According to the commonly accepted traditional account, the castle was built in 1193 by Masuda Kanetaka during the Kamakura period.
During the Nanboku-cho period (1336–1392), the castle became involved in military conflicts, and references to these events remain in historical documents associated with the Masuda family.
Major Renovations during the Warring States Period
During the later Sengoku period, the Masuda clan temporarily came into conflict with Mori Motonari, one of the most influential warlords in western Japan.
Amid these tensions, the clan leader Masuda Fujikane is believed to have significantly strengthened Nanao Castle and moved there with his retainers. This suggests that the castle was not simply a temporary refuge during wartime but also functioned as a place where people lived and conducted their daily affairs.
The Masuda clan eventually submitted to the Mori clan. Following the Battle of Sekigahara in 1600, the Masuda family was transferred to Nagato Province, and Nanao Castle was subsequently abandoned.
Although the castle itself disappeared, its remains provide valuable evidence of the political, military, and social history of medieval Iwami.
An Advanced Defensive Castle Built into the Landscape
Rows of Dry Moats on the Eastern Slope
One of Nanao Castle’s most distinctive characteristics is its emphasis on defending the eastern side of the mountain.
On the southeastern slope below the main bailey, 17 rows of ridge-and-furrow dry moats were arranged to interfere with enemy movement. A further seven were placed around the northeastern outer fortification.
The castle did not rely on the massive stone walls commonly associated with later Japanese castles. Instead, its builders made sophisticated use of the mountain’s natural contours, creating a defensive system based largely on earthworks, slopes, ridges, and moats.
Natural Features Used as Defensive Lines
The former river channel at the foot of the mountain is thought to have functioned as part of the castle’s defensive system. The surrounding wetland would have created an additional natural obstacle for approaching forces.
This combination of natural terrain and artificial fortifications demonstrates the ingenuity of Japanese mountain-castle construction during the medieval period.
The Connection with the Miyake Odoi Residence
Two Sites with Different Roles
Nanao Castle had a close relationship with the Miyake Odoi, the Masuda clan’s residential and administrative base during peacetime. The two sites are approximately 870 meters apart.
During peaceful periods, political affairs and daily life were centered at Miyake Odoi. In times of conflict, the clan and its retainers could retreat to the more defensible Nanao Castle.
Miyake Odoi occupied a substantial area measuring approximately 190 meters from east to west and 110 meters from north to south. Water-filled moats and earthen ramparts surrounded the residence, giving it the character of a fortified medieval estate.
Today, the site has been developed as Odoi Hiroba, where the arrangement of buildings from the medieval period has been recreated based on archaeological excavation findings.
Walking Through History and Nature
A Historic Hiking Experience
The Nanao Castle Ruins are now enjoyed as a scenic hiking area surrounded by greenery. Walking along the mountain ridge allows visitors to explore the remains of former castle enclosures, moats, and other defensive features.
Rather than seeing a reconstructed castle tower or large stone walls, visitors encounter the landscape itself. The slopes, ridges, earthworks, and traces of moats reveal how medieval Japanese castles were designed to take advantage of the natural terrain.
The summit offers particularly impressive views over the Masuda Plain and the Sea of Japan. The combination of panoramic scenery and historic remains makes the climb rewarding for travelers interested in both nature and Japanese history.
The Former Main Gate at Iko-ji Temple
A Surviving Structure Linked to Nanao Castle
One of the castle’s most intriguing surviving elements is believed to be the former main gate of Nanao Castle. According to tradition, the gate was moved after the castle was abandoned and is now preserved as the main gate of Iko-ji Temple.
The gate is designated as a Shimane Prefectural Cultural Property and provides a rare opportunity to see a historic structure associated with the former castle outside the castle site itself.
Why Visit the Nanao Castle Ruins?
Discover Medieval Japan Beyond the Famous Castles
Nanao Castle Ruins offer a different perspective on Japanese castle history from the famous castles with imposing keeps and stone walls.
The site reveals how medieval mountain castles were integrated into the landscape, using ridges, steep slopes, dry moats, earthen ramparts, and natural waterways as part of their defenses.
By visiting Nanao Castle together with the Miyake Odoi Ruins, travelers can gain a deeper understanding of how the Masuda clan organized its territory, combining a residential and administrative base with a fortified mountain stronghold.
A Historic Landscape Preserving the Spirit of Medieval Iwami
Nanao Castle Ruins are more than the remains of a former fortress. They are an important cultural heritage site that preserves the history of the Masuda clan and provides insight into the political, military, and everyday life of medieval Japan.
Surrounded by peaceful mountain scenery, the site allows visitors to walk through the remains of the castle while imagining the warriors, residents, and leaders who once lived and fought here.
For travelers exploring Masuda City, visiting Nanao Castle together with the Miyake Odoi Ruins and other historic sites connected with the Masuda clan is an excellent way to discover a lesser-known chapter of Japan’s medieval history.