A fortress was first built on this site in 1626, but a Swedish invasion in 1659 exposed its many weaknesses, prompting Frederick III to rebuild the defenses. The last building, known as the Kastellet (Citadel), was a five-pointed star-shaped fort that was finished in 1664. High embankments and a sizable, deep moat around it.
It was largely destroyed and reconstructed in the 19th century so that it could be used as a jail. Prisoners could attend Mass while remaining hidden from view by gazing through tiny viewing holes drilled into the walls of the cells, which were constructed up against the church. German soldiers occupied it during World War II, constructing a bunker there, and used it as their command center.
Although the gardens and ramparts are freely accessible to tourists through two paths to the north and south, it is now in use by the Danish military. The Kastellet is the ideal location for a leisurely stroll where you can see the windmill and vibrant barracks because of the unexpectedly calm mood inside the walls. Military drills may break up the stillness, and you could witness the mounted Guard Hussars dressed in full parade regalia.