Kōkokuji Castle Ruins: The Earthwork Fortress Where Hōjō Sōun Began
On a low spur reaching down from the southern flank of Mt. Ashitaka, in the Negoya district of Numazu, the ground still holds the shape of a fifteenth-century castle. There is no keep, no painted hall, no reconstructed gate. What survives at Kōkokuji Castle is the earth itself: a steep earthen rampart enclosing the main bailey, a dry moat cut deep across the ridge behind it, and a raised platform where a tower once stood. The site was designated a National Historic Site on March 17, 1995, and the area under protection was widened three times afterward, in 2000, 2007, and 2012.
The castle owes its fame to one man. This was the base from which Ise Sōzui, better known by his later name Hōjō Sōun, took his first independent step as a lord. The Late Hōjō clan, which would rule the Kantō plain for a century, traces its origin to this hillside.
From a Retainer's Reward to a Warlord's Starting Point
The man history calls Hōjō Sōun began as Ise Shinkurō, a samurai of modest standing. His sister had become a concubine of Imagawa Yoshitada, the military governor of Suruga Province, and through that connection he attached himself to the Imagawa house. When Yoshitada died suddenly, the family fell into a dispute over the succession. Shinkurō backed the young heir Imagawa Ujichika and helped settle the quarrel.
His reward came in 1488, when he received twelve villages of Shimokata estate in Fuji District and became lord of Kōkokuji Castle. The exact year the castle was first built is not known, and some historians have even questioned whether the stronghold he originally held stood on this precise spot. What the records agree on is the meaning of the moment: a man with no domain of his own now held a castle and land.
He did not stay long. In 1491 he crossed into neighboring Izu Province, destroyed the shogunal deputy Ashikaga Chachamaru, and made himself ruler there. He built a new headquarters at Nirayama Castle and moved on. From a borrowed foothold in Suruga, he had become a territorial lord in his own right.
Kōkokuji Castle did not fade after he left. In 1549 Imagawa Yoshimoto relocated the temple that had stood on the grounds and enlarged the fortress. After Yoshimoto's death, the castle changed hands repeatedly. From 1568 it passed to the Late Hōjō, who treated it as a frontline post against the Takeda and repelled a Takeda assault in 1571. When the Hōjō and Takeda made peace that same year, the castle was handed to the Takeda side and held by their kinsman Anayama Baisetsu. After Takeda Katsuyori fell in 1582 it came to Tokugawa Ieyasu, and a series of his vassals held it in turn. The last lord, Amano Yasukage, governed here until the castle was abandoned in 1607.
Why the Site Is Protected
Two things make Kōkokuji Castle worth preserving as a national monument. The first is its place in the story of the Sengoku period. A castle linked to the rise of Hōjō Sōun carries unusual historical weight, because his career is often taken as a marker for the age of warring states itself.
The second is the condition of the ground. Many castles of this era survive only as a name on a map. Here the earthworks remain legible. The rampart, moats, and tower platform sit close to what old castle drawings record, including a plan held in the Asano family archive and another preserved in the regional gazetteer Suruga Zasshi. Because the documentary record and the surviving terrain match so well, the site is treated as essential evidence for understanding how a Sengoku border castle was laid out and used. The protected area covers the main bailey, the cannon platform, the second bailey, a northern enclosure, and part of the earthworks of the third bailey.
What to Look For When You Visit
Kōkokuji is a hira-yama-jiro, a castle built on a low rise rather than a mountain peak. The enclosures run from south to north along the spur: third bailey, second bailey, main bailey. A car can reach as far as the second bailey, and the whole site can be walked in about half an hour.
The main bailey sits at an elevation of about 36 meters and measures roughly 60 meters east to west and 50 meters north to south inside its earthen wall. Stand below that wall and the scale becomes clear. Visitors regularly describe mistaking the rampart for a natural hillside before realizing it was raised by hand.
At the highest point is the platform traditionally called the tenshudai, the base of the castle keep. An excavation in 1982 uncovered the foundation stones of two buildings here, one to the east and one to the west, each about seven by eight meters. Sections of stone walling also remain at the platform, a feature worth pausing over since stonework was still uncommon in castles of this date.
Behind the main bailey, to the north, lies the feature many visitors find most striking: a great dry moat carved across the ridge to cut the castle off from the high ground beyond. It is deep and broad, and when the grass is low its shape reads clearly. East of the main bailey was the ishibiya-dai, a platform for an ishibiya, an early type of cannon. From the western tower platform the view opens toward Suruga Bay and, on a clear day, the Izu Peninsula.
Within the main bailey stands Homi Shrine. Despite its position, the shrine has no connection to the castle's military past. It was founded after the Ansei earthquake of the mid-nineteenth century, when local farmers, hoping for good harvests, enshrined an agricultural deity brought from a shrine in what is now Yamanashi. Beside it stand two stone monuments, one to Hōjō Sōun, the first lord, and one to Amano Yasukage, the last.
Around the Castle
Kōkokuji Castle sits in a landscape that explains why it was built where it was. To the south lay Ukishimagahara, once a belt of marshland so soft that men and horses could barely cross it, a natural barrier guarding the castle's flank. The old Nekata Kaidō, the road running along the foot of Mt. Ashitaka, passes through part of the site, and a southern branch road once linked it to the Tōkaidō, the great highway near the coast. The castle controlled a junction.
Numazu today is an easy base for a wider visit. The city faces Suruga Bay, with a working fishing port known for fresh seafood, and the long pine-lined shore of Senbon Matsubara nearby. Mt. Ashitaka rises behind the town for those who want a half-day hike, and the hot-spring resorts and castle sites of the Izu Peninsula lie within reach to the south.
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Q&A
- Is there a castle keep or building to see?
- No. Kōkokuji Castle was abandoned in 1607, and nothing was rebuilt. The appeal of the site is the earthworks themselves: the ramparts, the moats, and the keep platform. Visitors interested in original buildings should adjust their expectations; those interested in castle design will find a great deal here.
- How do I get there by public transport?
- The nearest stations are Hara and Numazu on the JR Tōkaidō Main Line. From either, a local bus runs toward the Higashi-Negoya stop near the site. Hara Station is also within roughly a 30-minute walk on flat ground.
- How long should I plan for a visit?
- About 30 to 40 minutes is enough to walk the third, second, and main baileys and climb to the tower platform. Allow extra time if you want to study the great dry moat at the rear, which is the highlight for many visitors.
- When is the best time to go?
- The site is open ground and can be visited year-round. The earthworks and moats read most clearly from late autumn through early spring, when the summer grass has died back and the shapes of the rampart and ditches are easier to follow.
- Is this one of Japan's recognized castles?
- Yes. Kōkokuji Castle is listed among the Continued 100 Fine Castles of Japan. The commemorative stamp is kept in a box beside Homi Shrine in the main bailey and can be used at any time.
Basic Information
| Name | Kōkokuji Castle Ruins (興国寺城跡) |
|---|---|
| Location | Negoya and Aono, Numazu City, Shizuoka Prefecture |
| Designation | National Historic Site |
| Date designated | March 17, 1995; protected area expanded in 2000, 2007, and 2012 |
| Castle type | Hira-yama-jiro (castle on a low rise) |
| Main bailey | Elevation about 36 m; roughly 60 m (east–west) by 50 m (north–south) inside the earthen wall |
| Period | Held from the late 15th century; abandoned in 1607 |
| Access | About 30 min on foot from Hara Station (JR Tōkaidō Main Line), or by bus from Hara or Numazu Station to the Higashi-Negoya area |
| Admission | Free; open site, no fixed hours |
References
- Database of National Cultural Properties, Agency for Cultural Affairs — Kōkokuji Castle Ruins
- https://kunishitei.bunka.go.jp/bsys/maindetails/401/1384
- Kojodan — Kōkokuji Castle
- https://kojodan.jp/castle/506/
- Shirobito — Kōkokuji Castle, the launching point of Hōjō Sōun
- https://shirobito.jp/article/889
Last verified: 2026.05.25