Akershus Fortress is a medieval castle built in the late 13century to protect Oslo, and hosts two specifically interesting museums for Norwegian defence history.
One of these are the Norwegian Defence Museum, located in the old Arsenal building inside Akershus Fortress since 1860. The museum has permanent exihibitions covering the time from the 15 century up till this date. Entrance to the museum is free, and there is a small cafe inside.
NB: Free entrance for everyone