Goryokaku is a former fortress, the first western type castle built in Japan in the late Edo period.
It is located in Hakodate city, one of the major tourist destination not only in Hokkaido but also throughout Japan.
Goryokaku was built to defend Hokkaido(then was called Ezo) from Russian invasion in middle of 19th century, At that time Ezo was homeland of mostly Ainu people which has distinguished history and culture than Japanese in Honshu island. Ainu were mostly hunters and Japanese started to trade with them in 16th century and the first trading post was built west of Hakodate in Matsumae.
Then, Hakodate became a very important port, one of the five ports opened by Tokugawa shogunate in middle of 19th century to the west after 250 years of isolation. During the isolation, Japan was contacting and trading with China, Korea and the Netherlands.
Hakodate was once the biggest city in Hokkaido and was the battlefield of Hakodate civil war between Tokugawa shogunate's remnants or patriots and new government. Toshizo Hijikata of former vice-chief commander of famous Shinsengumi(special elite force of samurai which hunted down terrorists in Kyoto during Tokugawa shogunate) led the rebellion army but it surrendered to the new government's modern army. Hijikata was killed in the action.
You can enjoy the Goryokaku's view by climbing Goryokaku tower which is nearby the fortress.