Akershus Fortress or Akershus Castle is a medieval castle in the Norwegian capital Oslo that was built to protect and provide a royal residence for the city. Since the middle ages the fortress was the namesake and centre of the main fief and later main county of Akershus, which was originally one of Norway’s four main regions and which included most of Eastern Norway. The fortress itself was located within Akershus main county until 1919, and also within the smaller Akershus sub county until 1842. The castle has also been used as a military base, a prison and is currently the temporary office of the Prime minister of Norway.

More details: https://en.wikipedia.org/wiki/Akershus_Fortress