Kastellet — the Citadel — is a star-shaped fortress built in the 17th century on the northern edge of the medieval city, facing the harbour. Five pointed bastions fan out from a central parade ground, surrounded by a moat that is still filled with water. The earthwork walls, windmill, and red-brick buildings within the walls are beautifully preserved.
The fortress is open to the public and makes for a pleasant walk from Nyhavn or the Little Mermaid. The paths along the ramparts give views over the harbour and across to the Swedish coast. The church within the walls is still active, as are some of the barracks. It is a tranquil, underrated spot that most visitors walk through quickly on the way to the Little Mermaid.