There isn’t a lot left of the old fortification around Copenhagen. The main remnants of the fortification are the old citadel Kastellet at the northern edge of the old town and Christianshavns Vold protecting Christianshavn against attacks from the south.
There used to be 11 towers on the old fortifications – but none of these has survived. The only tower which has been partly preserved is Jarmers Tower. The tower is named after the Jaromer who was the duke of the Wends. He attacked Zealand and Copenhagen in 1259. During his attack on Copenhagen he managed to break the old fortifications right at the spot where the tower has later been built. The Wends destroyed the city by burning the houses and demolishing the old castle of Copenhagen.
After the destruction the city was rebuilt and the fortifications were enforced by towers and one of them got the name after the former destroyer of the city. In 1607 the fortifications of the city was improved once again – and the tower was covered within the new fortification. The tower disappeared from view for centuries until the fortifications were torn down in 1884 and the tower once again became visible.
You find the Jarmers Tower on Jarmers Plads (square) which is located a short walk along the busy H.C. Andersen Boulevard from City Hall Square (Rådhuspladsen). The tower is located on the middle of a busy street but you can just cross to the middle and have a closer look at the remains of the tower. There are also some lions in front of the tower – I don’t think they have any special relation to the tower.