Kastellet is a small citadel located on the islet Kastellholmen. The first fortification on the location was built in 1667 designed by Erik Dahlbergh. It exploded in June 1845 and subsequently a new was built 1846-1848 to the design of architect Fredrik Blom. It consists of a round tower with red brick walls and a 20 meter high stair tower. On the top flies the Military Ensign of Sweden, it is hoisted and lowered every day, indicating the nation is at peace.
On May 17, 1996, the Norwegian Constitution Day, some Norwegian expats raised the Norwegian flag in the tower. Though such an action would historically have been taken as a declaration of war, a diplomatic crisis could be avoided.
