Kastellet, also known as The Citadel, is a well-preserved fortress and military complex from the 17th century in Northern Europe and is known for its distinctive star-shaped layout. Kastellet has two well-preserved gates, the King's Gate and the Norway Gate. The King's Gate, facing the city, is adorned with the monogram of Christian IV. The fortress also features drawbridges, moats, and other defensive elements. Inside Kastellet, there are various historic buildings, including military barracks, officers' residences, a church, and a windmill. 

Especially the windmill that was built in 1847 is a good picture spot.

Kastellet is open to the public and free to visit but it still houses military activities, and the area is used for ceremonial events and official functions. The gates are open from 6am to 10pm daily.