Monday, September 21, 2020

Ogasayama Fort -Tightrope for great leap as warlord-

 Ogasayama Fort

-Tightrope for great leap as warlord-



Name: Ogasayama Fort (Ogasayama-Toride)
Place: Iriyamase Kakegawa city,  Shizuoka
Location: 34.736956078382484, 138.01097867099892
Type: Mountain Castle
Built: 1568?
Remaining remnants: Clay walls and dry moats 

Brief History

Ogasayama fort (小笠山砦) is located over the peak area of Ogasayama mountain, one of about 260 meter above sea level, at 5 kilometer southward of current Kakegawa city. Ogasa Kyuryo hills to which Ogasayama mountain belongs is not so high but spreads over 10 kilometer long square, from Fukuroi city to Kakegawa city and Kikugawa city.

Ogasa Kyuryo hills are formed by rise of gravel layers including numerous small rocks. As the layer was not so pressed and hardened, it is easy to be eroded and make cliff. Contrary to the westward and southward from the peak gently descend to the hillside, the northward and eastward become the line of sheer cliff with numerous small valley. 

Takatenjin castle (Shizuoka prefecture) at the southeastern edge of Ogasa Kyuryo hills utilizes this sheer cliff of the east side of the hills. At westward, Yokosuka castle (Shizuoka prefecture) stands the edge of long ridge spreads from the peak toward southwest over 5 kilometers, and Hatta-san Soneiji temple also exists the west edge of the hills.

Situation around Ogasayama mountain

Originally at the peak of Ogasayama mountain there was Ogasa-Shrine which is told to be established about 1,300 years ago. Moderate height of mountain gathers many ridges from four directions and has a good scenery is suitable place for worship, especially as a guard of Tokaido road opened by ancient dynasty passed north side of the hills.

In the beginning of 16th century, Imagawa clan which was the warlord of Suruga province (middle part of Shizuoka prefecture) expanded its territory to Totomi province (west part of Shizuoka prefecture). For newly achieved territories Imagawa clan placed their retainers for administration.

At Kakegawa castle (Shizuoka prefecture), Imagawa clan placed their important retainer Asahina clan as the commander of the castle. For Mamuseduka castle at the west of Ogasa Kyuryo hills, Ogasawara clan which was originally a branch family of that clan at Shinano province (Nagano prefecture) but left the province after internal conflict was appointed.

Fall of Imagawa clan

For Takatenjin castle at the southeast of the province, Kushima clan which was another important retainer of Imagawa clan was appointed. But Kushima clan suffered severe damage at the intrusion into Kai province (Yamanashi prefecture) and fell at the internal conflict of Imagawa clan in 1537, called as “Hanagura-no-Ran”. Yoshimoto Imagawa (1519-1560), the leader of Imagawa clan, placed Ogasawara clan at Takatenjin castle.

Imagawa clan experienced its peak period under Yoshimoto Imagawa, and expanded its territory toward the border of Mikawa province (east half of Aichi prefecture) and Owari province (west half of Aichi prefecture). But in 1560 Yoshimoto Imagawa died at the battle of Okehazama before Nobunaga Oda (1534-1582), the warlord of Owari province.

Upon the death of Yoshimoto Imagawa, Ieyasu Tokugawa (1543-1616) who subordinated to Imagawa clan became an independent warlord and allied with Nobunaga Oda. Ieyasu Tokugawa raised his army against Imagawa clan, and seized Mikawa province by 1566. Ujizane Imagawa (1538-1616), successor of Yoshimoto Imagawa, tried to stop the movement of Ieyasu Tokugawa but could not stop it.

Siege of Kakegawa castle

In 1568, Ieyasu Tokugawa allied with Shingen Takeda (1521-1573), the warlord of Kai province and former ally of Yoshimoto Imagawa, and both lords attacked the territory of Imagawa clan at once. Takeda army captured Sunpu city, the capital of Imagawa clan, and Ujizane Imagawa escaped to Kakegawa castle kept by his retainer Yasutomo Asahina (?-?).

At the same time Tokugawa army entered into Totomi province and captured Hikuma castle (Shizuoka prefecture, later Hamamatsu castle). Next Ieyasu Tokugawa proceeded to the east half of the province and encircled Kakegawa castle where Ujizane Imagawa besieged. Ieyasu Tokugawa negotiated with other retainers of Imagawa clan, and Ogasawara clan turned to Tokugawa clan.

However, Yasutomo Asahina who accepted his lord showed brave fight at Kakegawa castle and kept it for several months. Ieyasu Tokugawa had to prepare the attack of former Imagawa retainer from backward, and also the movement of Takeda army once returned to Kai province but showed the sign of invasion to Totomi province. Preparing for endurance battle, Ieyasu placed his headquarter at the top of Ogasayama mountain where Kakegawa castle is looked down.

Structure of Ogasayama fort

Structure of Ogasayama fort spreads with ridges of Ogasayama mountain, utilizing narrow point as bottleneck and wide point as residential area. As a temporal military camp the central area is not clear but it might be the terrace of about 50 meter long and 20 meter wide, that is told as the place of “Sasagamine Goten” residence used by Ieyasu.

Westward of central area is protected by the layers of terraces built at small peak, that has a combination of dry moats and buffer area at the front side. Ahead of this area there are two line of terraces, one spreads northwestward toward the peak of the mountain and another is over 200 meter line of terraces that edge is protected by clay wall and dry moats.

Eastward of central area might be the front side of the fort. This part is also protected by the small peak fortified by line of horizontal dry moat and sheer cliff. Ahead of this part there are two long and narrow terraces, and from the terraces used as the ground of Ogasa Shrine the shape of Takatenjin castle is well seen. Total size of the castle is about 500 meter long, and quite large one as a military camp.

Conflict with Takeda clan

As a result, Kakegawa castle opened after five month battle in 1569. Ujizane Imagawa exiled to Hojo clan which was the warlord of Sagami province (Kanagawa prefecture), and Ieyasu Tokugawa seized whole part of Totomi province. But at the same time Shingen Takeda captured whole part of Suruga province and next aimed at Totomi province.

In 1571, Shingen Takeda intruded into Totomi province and attacked Takatenjin castle but could not fall it and retreated. Next year Shingen made total attack to Tokugawa clan and broke Tokugawa army at the battle of Mikatagahara, but next year died in ill during the operation. Takeda army retreated to Kai province then Ieyasu Tokugawa left the crisis,

After two years of the death of Shingen Takeda, his successor Katsuyori Takeda (1546-1582) started operation against the ally of Ieyasu Tokugawa and central ruler Nobunaga Oda again. In 1574, Katsuyori attacked east part of Mino province (Gifu prefecture) and fell several castles of Oda clan, then next attacked Takatenjin castle and fell it. 

Change of situation

Fall of Takatenjin castle that his father Shingen Takeda could not fall increased the reputation of Katsuyori. Next year Katsuyori continuously attacked Mikawa province that is the origin of Tokugawa clan, and captured several castle. But in June Katsuyori suffered severe defeat at the battle of Nagashino from Oda and Tokugawa armies.

Now the situation has changed then Ieyasu Tokugawa turned to the offensive toward Takeda army. Tokugawa army recovered Futamata castle (Shizuoka prefecture) close to their main base Hamamatsu castle (Shizuoka prefecture), and pushed out Takeda army from Mikawa province. 

Further Tokugawa army captured Suwahara castle where is an entrance of Totomi province from Suruga province. By capturing Suwahara castle Takeda army could not support Takatenjin castle from land, but as Takatenjin castle had an inlet and sea port at that time then Takatenjin castle still stood by Takeda army.

Siege of Takatenjin castle

Now the biggest goal of Tokugawa army became Takatenjin castle, to recover the remaining territory of Totomi province. In 1576 Ieyasu Tokugawa constructed  Ogasayama fort again, to watch Takatenjin castle and supply route of Takeda army from Oyama castle (Shizuoka prefecture) near the coast. Next Tokugawa clan built Yokosuka castle at the coast, to prevent the intrusion of Takeda army along the coast.

Next during 1578 to 1580, Ieyasu built six forts at the east of Takatenjin castle to prevent the supply of Takeda army utilizing the ship. Motonobu Okabe (?-1581), the commander of Takatenjin castle, requested reinforcement to Katsuyori but Katsuyori faced Hojo army at Kanto region and could not send support to Takatenjin castle.

Finally in 1581, the supply of Takatenjin castle was exhausted, then Takeda army charged to surrounding Tokugawa army but almost generals and soldiers died in the battle. After five year siege Tokugawa clan could recover whole territory of Totomi province, then Ogasayama fort and other forts lost their role and were abolished.

Afterward of castle

Capture of Kakegawa castle was the turning point for Ieyasu Tokugawa to progress from single province lord to multiple province lord. Seizure of Takatenjin castle was a long and tough battle, but the fact of defeating Takeda army only by themselves opened the door for Tokugawa clan to the major power during the collapse of Takeda clan and death of Nobunaga Oda. From that point Ogasayama fort was an important military camp for Tokugawa clan.

Now no building remains but structure of the castle such as dry moats or clay walls well remain around the climbing road. Of course different from permanent castle, but defense structures of the fort are well constructed show the strong will of Tokugawa clan to fall target castles. Narrow path of the fort faces cliff shows the tight rope of Tokugawa clan, that Tokugawa clan could pass it and made a leap to large warlords.


30 minutes drive from Tomei Expressway Kakegawa interchange to parking of Ogasa Jinja Shrine. Be careful for narrow path next to sheer cliff fallen to die.

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Pictures (click to enlarge)

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