The fort was built in 1777-1780 by the Danish, who originally named it Prince Frederik's Battery.

[1] At that time, Hassel Island was part of the Danish West Indies.

[2] Originally, the fort consisted of parapet wall, gun platform, barracks, magazine, and storeroom, as well as kitchen, cistern, and privy.

[3] Today, the remains of the fort consist of the remnants of the magazine, barracks, and cistern.

[4] Fort Willoughby is protected within the Virgin Islands National Park.