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.
King Christian IV of Denmark initiated Kastellet’s construction in 1626 with the building of an advanced post, the Sankt Annæ Skanse (St. Anne's Redoubt), on the coast north of the city. The redoubt guarded the entrance to the port, together with a blockhouse that was constructed north of Christianshavn, which had just been founded on the other side of the strait between Zealand and Amager. At that time the fortifications only reached as far north as present day Nørreport station, and then returned south east to meet the coast at Bremerholm, the Royal Shipyard. However, part of the king's plan was to expand the area of the fortified city by abandoning the old East Rampart and instead extend the rampart straight north to connect it to Sankt Annæ Skanse. This plan was not completed until the mid-1640s, shortly after King Frederick III succeeded King Christian IV.
After the Swedish siege on Copenhagen (1658–1660) the Dutch engineer Henrik Rüse was called in to help rebuild and extend the construction. The fortification was named Citadellet Frederikshavn ('The Frederikshavn Citadel'), but it is better known as Kastellet ('the citadel').
Kastellet was part of the defense of Copenhagen against England in the Battle of Copenhagen (1807). During the German invasion of Denmark on 9 April 1940, German troops landing at the nearby harbor captured The Citadel with very little resistance, thereby forcing the Danish government to surrender.
Kastellet was renovated 1989–1999 with funds from the A.P. Møller and Wife Chastine McKinney Møllers General Fund.
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 its mainly serves as a public park and a historic site.
References:Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.