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.
There are only two roads into Kastellet, if you exclude trying to scale a wall - the gates facing North and South. This is the South gate, through which the characteristical buildings of Kastellet can be seen. Today, Kastellet houses both Army and Home Guard functions, but still have a contingent of soldiers guarding the fortress, and parading around it every evening before the flag is lowered.