Kastellet, located in Copenhagen, Denmark, is one of the best preserved fortresses in Northern Europe. It is constructed in the form of a pentagon 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 the Kastelskirken as well as a windmill. The area houses various military activities but it mainly serves as a public park and a historic site.

More details: https://en.wikipedia.org/wiki/Kastellet,_Copenhagen