Akershus Castle and Fortress is Oslo's medieval stronghold, founded in 1299 and still standing above the harbour close to the city centre. Its stone walls, courtyards, narrow passages, and stairways are open to wander, with strong views over the Oslofjord and the waterfront.
The fortress area also contains museums tied to Norwegian military history, making it one of the easiest major historic stops to add to a walk between the harbour, city hall, and Bjørvika.