The Citadel in Copenhagen was founded by Danish King Christian 4 in 1626. You will find it on the border of inner Copenhagen and the area of Østerbro. Today, the buildings inside the Citadel are used as military barracks and offices. However, the area is open to the public and a favourite green lung for a brisk walk or jog on the ramparts. https://www.visitcopenhagen.com/copenhagen/citadel-gdk410745