One of the best preserved fortresses in Northern Europe. It is constructed in the form of a pentagon with bastions at its corners. The Citadel was founded by Danish King Christian 4 in 1626.It was completed until the mid-1640s, shortly after King Frederick III succeeded King Christian IV. Today, the buildings inside the Citadel are used as military barracks and offices. However, the area is open to the public. Where: Adress is Kastellet 30, Copenhagen Østerbro. What to see next: