hokubeihochi:

Fort Goryokaku (五稜郭, Goryōkaku) is a massive, star shaped, Western style citadel, which was built in the last years of the Edo Period for the defense of Hakodate against the imperialist threat posed by the Western powers. A few years later, the fort became the site of a civil war between an army of the shogunate and the superior troops of the newly established Meiji government.

After the fort had lost its military importance, it was eventually turned into a public park in the 1910s. Over one thousand cherry trees were planted along its moats, making it one of Hokkaido’s best cherry blossom spots. The peak of the blooming season usually takes place around early May.

[from japan-guide.com]