This medieval fortress has stood guard over Oslo's harbor for over 700 years. Today it serves dual purposes: a popular recreational area for locals and an active military base for the Norwegian Armed Forces. The complex splits into two parts with different access rules. The Fortress (Festningen) refers to the entire walled area including parks, ramparts, and exterior courtyards, free to enter and generally open 6am-9pm daily. The Castle (Slottet) is the specific medieval building inside the walls where the King and Queen host official dinners, requiring paid admission with limited hours.
The ramparts offer one of Oslo's best sunset views from the bastion facing the water, looking out over Aker Brygge and the fjord islands. Old cannons still point toward the water and fire salutes on Royal birthdays and Constitution Day (May 17th). The entire fortress is paved with rough uneven cobblestones. Don't wear high heels. Strollers and wheelchairs need to stick to main paved paths since side routes are difficult to navigate.
Inside the paid Castle admission, you see the Royal Mausoleum (resting place of modern Norwegian kings and queens: Haakon VII, Queen Maud, Olav V, and Crown Princess Märtha), dark cramped dungeons where famous prisoners were held, and austere banqueting halls that reflect Norway's historically modest court compared to continental European palaces.
Norway's Resistance Museum sits in a 17th-century building on the fortress grounds, detailing the Nazi occupation (1940-1945) through dense atmospheric exhibits using sound and lighting to recreate the oppressive feeling. The collection of homemade radios and secret weapons used by Milorg resistance fighters is the highlight. The Norwegian Armed Forces Museum occupies an old arsenal building near the cruise ship quay, covering Viking battles to modern peacekeeping missions with free admission and massive exhibits including tanks and airplanes suspended from the ceiling.
The fortress is rumored to be Norway's most haunted place. Malcanisen (The Vicious Dog) is a ghost legend about a live dog buried in the foundation to protect the fortress. Seeing it is considered a death omen—you'll die within three months if you look into its eyes according to local superstition. The Faceless Woman appears in a long robe near Margaretasalen, associated with a former maid who died tragically.