Kastellet is a castle, one of the forts star the one built to withstand gunfire, best preserved in Europe. Even today, some of its parts and buildings are used by the military and spies in Denmark. But since the Danes have changed its nature (the joys of living well) today Kastellet has become one of the most popular parks in Copenhagen.
You can walk in the paths of the citadel or climb the strengthened ramparts to look at the sea; it has become normal for the people who go  there in order relax. Relaxation is not lacking even for tourists, but they can find an opportunity to take some pictures of the ramparts, features of red barrack or only the mill that is inside. Not to mention that the Kastellet is not so far from the Little Mermaid.
Photo 1:Wikimedia Commons Photo 2:Edlondon29, Photo 3: Ma Rui
