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Castle Koyama Castle (Shizuoka)

This article is in the series Walking the Japanese Castles
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Koyama Castle (小山城) is a Sengoku-era castle in former Tōtōmi Province. It is located in modern-day Yoshida City, Shizuoka Prefecture, and is a designated Historic Site.

Koyama Castle (小山城)


Although no details are known, the castle was likely built on the foundations of a fort named Yamazaki (山崎の砦) erected by the Imagawa clan. After Imagawa Yoshimoto (今川義元, 1519-1560) was defeated in the Battle of Okehazama, Takeda Shingen and Tokugawa Ieyasu agreed to divide his territories: Shingen would take Suruga and Ieyasu Tōtōmi, with the Oi River (大井川) as their boundary. However, in December 1568, Shingen broke the agreement and defeated Yoshimoto's son, Imagawa Ujizane (今川氏真, 1538-1615), capturing Yamazaki. At the same time, Ieyasu took Kakegawa Castle and transferred the territory around the fort to Matsudaira Sanenori (松平真乗, 1546-1582) who captured it in April 1570.

Author's comment: at school, students of history are only taught the 'dots' of historical events, so they sometimes lose sight of the 'lines' that connect these events. Suppose they learn that "after Imagawa Yoshimoto was killed in battle, his successor Ujizane was so weakened that the Takeda and Tokugawa clans destroyed him". In that case, they get the impression that the Imagawa clan vanished right after Yoshimoto's death. Another eight or nine years passed until the Imagawa were defeated. Also, when we learn that "Nobunaga's artillery obliterated the Takeda cavalry in the Battle of Nagashino," we get the impression that the Takeda clan was defenceless in the face of the Oda forces. Although Oda prioritised the pacification of the Kinai and tackling the Tokugawa, the Takeda waged a fierce war against Ieyasu for another seven years. It is impossible to understand the conflicts raging in Tōtōmi and Suruga without looking at this time frame.

In February 1571, Shingen attacked Yamazaki with 25,000 men, took it from Sanenori and named it Koyama Castle. Ōkuma Tomohide (大熊朝秀 1517-1582) was placed in charge of the castle. Tomohide had initially been a vassal of Uesugi Kenshin but had left the clan during factional strife, entering Shingen's service. The Takeda reinforced Koyama Castle and integrated it into a defence line west of the Oi River which became a base for the Takeda to attack Takatenjin Castle (高天神城) in Tōtōmi Province. The capture put Shingen in a position to take Asuke (足助), Tashiro (田代) and Nireki (二連木) in Mikawa Province.

After Shingen's death, his son and successor, Takeda Katsuyori (武田勝頼, 1546-1582), built Suwabara Castle (諏訪原城) in Kanayahara along the Tōkaidō Highway as a foothold for an attack on Takatenjin Castle. However, after Ieyasu and Nobunaga defeated Katsuyori in the Battle of Nagashino (1575), Suwabara Castle fell, and the surviving Takeda troops fled back to Koyama Castle. Despite fierce attacks from the Tokugawa forces, Koyama Castle held out under Okabe Motonobu (岡部元信) and his men.

The castle protected a vital supply route to Takatenjin Castle on the Takeda's front line. In response, the Tokugawa forces decided to starve the defenders out by harvesting the rice paddies south of the castle. The skirmishes between the two armies continued, but when Takatenjin Castle fell in March 1581, the forces of Oda Nobunaga and Tokugawa Ieyasu began their conquest of Kai and Shinano in February 1582, Tomohide set fire to the castle and abandoned it.

The castle:

The topography of Koyama Castle reveals that it is surrounded by cliffs to the north, east and south. Furthermore, a river seems to have flowed on all three sides at the time. To the west, the only weak point, Takeda-style crescent-shaped moats and triple ditches, were built to strengthen the defences.

Koyama Castle (小山城)

A castle diorama displayed in the observation tower.

Koyama Castle (小山城)

Based on the Aerial Photograph Service of the Geospatial Information Authority of Japan. The location of the enclosures is estimated.

We visited the castle by car. The car park is at the foot of the mountain, in front of Nōmanji Temple.

① Nōmanji Temple (能満寺)
This temple is located at the foot of Jōyama, famous for its large cycads.

Koyama Castle (小山城)

Visitors need to climb these stairs.

Koyama Castle (小山城)

② Observation deck (展望台)

The observatory has the shape of a keep and houses a museum. The original castle had no keep.

Koyama Castle (小山城)

③ Crescent moat (三日月堀)

View from the observatory.

Koyama Castle (小山城)

We were lucky: Mount Fuji was visible.

Koyama Castle (小山城)

④ Triple moat (三重堀)

Another highlight is the triple moat. These defences helped repel Tokugawa attacks.

Koyama Castle (小山城)

⑤ Otemon (大手門)

The main entrance is opposite the car park.

Koyama Castle (小山城)

The remains of a Kuruwa are one level beneath that of the Otemon.

Koyama Castle (小山城)

A river runs at the foot of the mountain.

Koyama Castle (小山城)

Date of visit: 16 February 2014


  • Address: 2519 Kataoka, Yoshida, Haibara District, Shizuoka 421-0303
  • Access: two kilometres from Yoshida Interchange of Tomei Expressway via Prefectural Road 34.
  • Parking: Nōmanji Park car park (free of charge: point 'P' on the map above).
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Hiroto Uehara
Hiroto is an ordinary Japanese office worker, but his true mission is searching for castles on the weekend.


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Hiroto Uehara
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