Christiansholm Festning
This circular fortress with 16-foot-thick walls, on a promontory opposite Festningsgata, was completed in 1672. Its role has been much more decorative than defensive; it was used once, in 1807, during the Napoleonic Wars, to defend the city against British invasion. Currently, visitors can only explore the grounds as the inside fortress isn't open to the public, but a permanent exhibition on the grounds provides insight into the fortress and the city's history.