Kastellet is one of the best preserved star fortresses in Northern Europe. It is constructed in the form of a pentagram with bastions at its corners. Kastellet was continuous with the ring of bastioned ramparts which used to encircle Copenhagen but of which only the ramparts of Christianshavn remain today.

A number of buildings are located within the grounds of Kastellet, including a church as well as a windmill. The area houses various military activities but it mainly serves as a public park and a historic site.