Koga City is situated on the westernmost tip of Ibaraki Prefecture, at the confluence of Tone River in the south and Watarase River in the west. The town prospered as an important traffic point along the Nikko Highway. Koga Castle (古河城 Koga-jō) was built in the late Heian Era on the banks of the Watarase by Shimokobe Yukihira (下河辺 行平), a warrior of the Minamoto clan. In the wake of Minamoto no Yorimasa's (源頼政, 1106–1180) defeat in the Battle of Uji (1180) and his subsequent seppuku the Shimokobe brought Yorimasa's head to Koga where it was interred in a Shinto shrine.
In 1455, Ashikaga Shigeuji (足利成氏, 1438–1497), the last Kantō kubō (関東公方, deputies of the Ashikaga shogun who administrated the Kanto area) escaped to Koga from the forces sent by the shōgun, Ashikaga Yoshimasa. Shigeuji established his base there and assumed the title Koga kubō. Koga Castle played a significant role as the base of the Koga kubō during the Sengoku Period, until 1583, when the castle was taken over by the Late Hōjō. After the defeat of the Hōjō in the Siege of Odawara (1590), Toyotomi Hideyoshi awarded the Kantō area to Tokugawa Ieyasu who in turn assigned Koga Castle to his grandson-in-law, Ogasawara Hidemasa. Koga became the capital of Koga Domain, while the castle - ruled over by a cast of fudai daimyō (mainly the Doi, the Matsudaira, and the Ogasawara clans) - served as one of the defensive positions protecting Edo. Koga remained an important post town along Nikkō Highway, and the shōgun often stayed there on his way to visiting Nikkō Shrine.
Barely any remains of Koga Castle are left. The structure was dismantled in 1873 and parts of the former castle grounds were flooded when the course of the riverbed was changed in the Meiji period as a measure of flood control. It is ironic to think that the castle had prospered thanks to the river and was eventually destroyed by the river.
Arriving at Shin-Koga in Saitama I crossed Watarase River into Ibaraki. From Mikuni Bridge, it took fifteen minutes to reach the old gate of a former chief retainer’s mansion. It still seemed to serve as a private residence, as indicated by the nameplate on the front entrance.
The Shishigasaki Earthworks are some of the few remains of the former castle.
Then I proceeded to the Koga Historical Museum. The museum is located on the former castle grounds.
The museum hall holds an impressive diorama of Koga Castle displaying its former location floating in the middle of a huge pond. It also houses a considerable map collection of Takami Senseki (鷹見泉石, 1785-1858), a chief retainer of Koga Domain and a "Dutch scholar". Dutch, or rather, Western studies (蘭学 rangaku) referred to the studies of Western technology and medicine introduced to Japan by the Dutch merchants stationed in Dejima.
I continued along the Koga-kubo mansion path in Koga Park.
The site was surrounded by a partly restored moat, giving the visitor a vague impression of the original shape (clearer an idea at least than of what is left of the castle).
The old residences are on display, too. The gate wall of the samurai residences can still be seen in the northern part of the city.
（Date of visit: 6th June 2013）
- Address Chuocho, Koga-shi, Ibaraki Map
- Transportation (To Koga History Museum): 13 minutes from JR Tohoku line Koga station on foot, 25 minutes from Tobu Nikkou line Shin-Koga Station on foot
- Other sights The site of the Koga-Kubo mansion (in Koga Park), Koga City Walking Map (Japanese: Koga City Tourists Association HP)