Kristiansten Fortress was built in 1681 after the Great Fire of Trondheim. It was already well established as a foam in 1718 and since 1816 it no longer fulfils its original function. The site was temporarily used by soldiers of the German army during the Second World War.
If the flag flies over the fort (which can be seen from much of the city) then the fort is open to the public (usually every day). You can visit the armoury in which there is a museum exhibition and also the prison.