As far back as in 1626, an entrenchment was built by King Christian the Fourth in the Northern part of the defense wall of Copenhagen, in order to strengthen that section of the defenses. His second-oldest son, who succeded him on the throne, King Frederik the Third, decided to improve the defences on that spot even more, and "Kastellet" (Citadellet Frederikshavn) came into being. Finished in 1663, and located right behind "Den Lille Havfrue" (The Little Mermaid) and Langelinie, Kastellet was in its active days an almost self supplying fortification.

The buildings in Kastellet are to-day used as military barracks, but the area is open to thepublic and a favourite 'green lung' for a brisk walk on the ramparts.