Monday, September 23, 2019

Iwatsu Castle -Long journey to Pax Tokugawania (2) expand to Okazaki plain-

Iwatsu Castle

-Long journey to Pax Tokugawania (2) expand to Okazaki plain-



Name: Iwatsu castle (Iwatsu-jo)
Place: Iwatsucho Okazaki city, Aichi
Type: Hill Castle
Built: 16th century
Remaining remnants: Clay walls and dry moats 

Brief History

Continued from Part 1

Iwatsu castle (岩津城) is located over a hill of about 30 meter height between Yahagi-gawa river and  Tomei Expressway at Iwatsu town, in the north part of current Okazaki city. Castle site is the western edge of Mikawa Highland, and just at the point where Yahagi-gawa river flows hill area into flat area of Okazaki plain.

Now this place is totally inland area but in ancient era before land reclamation large humid area covered lower part of Yamagi-gawa river. Iwatsu are where has river terraces at both side of the river was a stable crossing point of the river, then this area was a crossing point of vertical river transportation and horizontal land transportation. 

On the opposite of the river there is a ruin of Kitano Temple, a huge ancient temple, and this shows the importance of Iwatsu area. Along with the time the crossing point of river moved downward to current Okazaki city central, but Asuke Kaido road toward Shinano province (Nagano prefecture) diverged near the castle site then still important as an entrance to mountain area.

Origin of Iwatsu castle

Precise year of construction is unknown but considering its geographical importance there might be a fort in early days. In the middle of 15th century, Nobumitsu Matsudaira (1404?-1488?), the lord of Matsudaira castle (Aichi prefecture) and who was recorded as grandson of Chikauji Matsudaira, acquired Iwatsu castle by occupation or purchase then used it as new main base.

Matsudaira clan which might be originally mere wealthy farmer emerged to the history at the period of Nobumitsu as a retainer of Ise clan, a high class magistrate of Muromachi Shogunate. At the middle part of Muromachi era, there was no strong governor at Mikawa province (eastern half of Aichi prefecture), then Shogunate directly managed the province through magistrates.

Originally Mikawa province was governed by Ashikaga clan which later established Muromachi Shogunate, then major branch families of Ashikaga clan such as Kira clan, Nitsuki clan, Issiki clan, Imagawa clan or Hosokawa clan held the land name of province. But at the establishment of Shogunate, these families became high class retainers of Shogunate then moved to Kyoto city.

At first Nitsuki clan stem from neighbor place of Iwatsu castle was appointed as the governor but later lost the position, then Issiki clan succeeded the position. But Issiki clan was also purged by Shogunate then Hosokawa clan became next governor but opposed to former Issiki retainers.

Furthermore, Kira clan which had higher authority than Issiki clan or Hosokawa clan because of the closeness to the Shogunate stayed as an independent power at coast area but held internal conflict. In such situation local lords of Mikawa province raised their army requesting their autonomy.

Growth by suppressing rebel

In response to such situation, Shogunate ordered the governor Hosokawa clan to suppress such rebel in 1458. Hosokawa clan gathered Matsudaira clan and Toda clan which was the local lord of Tahara castle at Atsumi peninsula, as major local lords of Mikawa province in west half and east half to attack opposing local lords. 

Around this time, Nobumitsu broke Saigo clan which held the central area of current Okazaki city and built Okazaki castle (Aichi prefecture) just before. Nobumitsu sent his son to the successor of Saigo clan and substantially took over it then seized whole part of Okazaki area and became the strongest local lord of Mikawa province.

As rebel lords were in the south part of current Okazaki city and Kota town, territory of Matsudaira clan further expanded southward at eastern side of Yahagi-gawa river. It is recorded that Nobumitsu had 48 sons and daughters, and placed their sons to newly achieved territory and let their daughter married with local lords then built up large family. Along with the growth of Matsudaira clan, Iwatsu castle might be expanded.

Structure of Iwatsu castle

Iwatsu castle spreads over the slope of the hill prolongs from its peak toward south. Central area of the castle is roughly square shaped one of 100 meter long, which has a line of clay wall at its southern line and front gate in the middle of this line. Remaining three directions are protected by layer of corridor area which might be added later to spare the low height of the hill.

At the south of central area, ahead of dry moat and clay bridge, front gate area which is a combination of two compartments encircled by clay wall. Originally it might be a large flat area same as central area, but later it became smaller by digging huge dry moat and piling up clay wall thus changed to gate area. By connecting two compartments, resilience for attack and flexibility of counter attack was brought.

According to old picture of the castle, there were several flat areas at the southwest of core area to hillside, which is used as a ground of houses. No structure is depicted on the south half of hill but it might be used as front fort in case of emergency. Total size of the castle is about 300 meter long and 200 meter wide, and its size and structure clearly shows development of Matsudaira clan.

Expand to Anjo area

In the meantime, the battle of Onin, a huge and long battle divided most part of major governors of Muromachi Shogunate, occurred in Kyoto city in 1467. At this time Matsudaira clan belonged to Eastern army which was lead by Hosokawa clan, but Hatakeyama clan at Anjo area which was the opposite of Yahagi-gawa river supported opposite Western army.

Utilizing this situation, Nobumitsu planned to proceed to Anjo area and attacked Anjo castle (Aichi prefecture). According to the record, Nobumitsu intentionally held festival near the castle, and guard of Anjo castle left the castle to see it and only noncombatant remained the castle. Looking at this Nobumitsu made sudden attack to the castle and fell it in 1471.

Nobumitsu could built the basis of Matsudaira clan consist of many families at various point, which brought internal conflict but could keep Matsudaira clan by flexible but stable structure. Capture of Anjo area opposite of Okazaki area also brought base of continuation even if losing one area.

Move of main base and afterward

Iwatsu castle had been the main base of Matsudaira clan for 40 years, but faced the invasion of Imagawa clan, the warlord of Suruga province (middle part of Shizuoka prefecture) also held Totomi province (west part of Shizuoka prefecture) in 1508. This Imagawa army was lead by Sozui Ise (1432-1519, later Soun Hojo), the founder of Hojo clan and was a retainer of Imagawa clan at this time.

At this time Matsudaira clan at Iwatsu castle suffered severe damage and fell. On the other hand, Nagachika Matsudaira (1473-1544), the third son of Nobumitsu and lord of Anjo castle, could preserve his army at the opposite of Yahagi-gawa river and could force Imagawa army to retreat. After this battle Anjo castle became the main base of Matsudaira clan.

After that Iwatsu castle might continue as the guard of north part of the territory. In 1583, when Ieyasu Tokugawa (1543-1616), a descendant of Nobumitsu Matsudaira, fought against central ruler Hideyoshi Toyotomi (1537-1598), Ieyasu and his ally Nobukatsu Oda (1558-1630) reformed major castles in their territory against dominant Toyotomi army. Huge dry moat and gate area of Iwatsu castle might be added this time.

In 1590, Ieyasu Tokugawa was moved to Kanto region by Toyotomi government, and Iwatsu castle might be abolished at this time. Now no building exists but structure of core area barely remains in bamboo forest. Outer area was lost by development and manage of core area is not clear but it is hoped to the remnant of the castle for the future.


As permission for entry is not completely clear even though sign board is placed, detail is omitted. Entrance is at own risk and careful behavior is required.

Continue to Part 3

Related Castles

Matsudaira Castle -Long journey to Pax Tokugawania (1) from small mountain village-
Anjo Castle -Long journey to Pax Tokugawana (3) change of main family-

Pictures (click to enlarge)