Tourist Copenhagen

Official tourist-site about CopenhagenWhat to see and do Copenhagen -: "

Copenhagen has three sea forts at the entrance to the harbour in the middle of Øresund:-
Middelgrundsfortet, Flakfortet, and Trekroner

They are all artificial islands with Middelgrundsfortet being the largest of its kind in the world and also the best preserved of the three."

Karl Dane is said to have served on Trekroner but I have not found any documentation