Standing on a hill to the city's east, Kristiansten Fortress (Kristiansten Festning) was built between 1681 and 1695 to protect the city against attack. It is a bit of a climb, but admission to the tower and its small museum is free, and the views across the city are lovely.
Grim reminders of the fortress's 20th-century history are the cells in which the Nazis held members of the Norwegian Resistance during the World War II occupation, and the memorial to those who were executed here.
