Most of the old fortification around Copenhagen was demolished in the 1840s but a little part has been preserved. It is called Kastellet and it was the main fortification protecting the northern end of Copenhagen and the access to the harbor in Copenhagen.
The old citadel was built by King Christian IV in 1624 to protect the expanded city of Copenhagen outside the original walls. He intended to build a castle inside the new fortification so the king could retreat there in case of danger – but by the time the area was ready for a castle there was no money left to actually build a castle so it has never been build.
Kastellet was a bit different from the rest of the fortification around Copenhagen since some of the bastions on the citadel actually face towards the city so the guns could be pointed towards the city itself and fire on the city in case of a revolt against the king. Fortunately the guns have never been turned to fire on the citizen of Copenhagen.
The citadel was used in the defense of Copenhagen against the Swedish sieges from 1658 to 1660 which ended in a spectacular Danish victory which but Denmark still had to sue for peace and give up very large possessions to the expansionist Swedish tyrant king.
The citadel also fought the British navy in 1807 during the battle of Copenhagen. The British wanted the Danish navy since it was the second biggest navy in Europe after the British navy and after the Battle of Trafalgar it was the only possible threat to the British navy anywhere in the world during the napoleon wars. Unfortunately the defenders were not successful this time around and after a rootless bombardment by the British navy the city surrendered and Denmark had to give its fleet to the British.
The battle of Copenhagen were the last major military action the citadel saw – during the German occupation of Denmark the German troops landing in the harbor not far from the citadel captured the citadel without resistance. The German army used the area as a prison for certain prisoners – and after the war Werner von Best the German commander of Denmark were shortly imprisoned here before he was transferred to other prisons including the prison museum of Horsens.
The citadel is still a military area and large part of the Danish army high command is located here including parts of the military intelligence and the Chief Advocate Corps and the Danish home guard.
Despite the military presence in the area it is still one of the most attractive places in central Copenhagen to go for a little walk in a green surrounding. You can walk around the area and look at the old buildings. One of the most interesting buildings is an old windmill inside the citadel.
It is free to go in and wonder around Kastellet and it is a good place to visit if you are walking from Amalienborg royal residence and the Little Mermaid since Kastellet is pretty much at the midway point of this short walk.