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 has been 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 the Akershus main county until 1919, and also within the smaller Akershus sub county until 1842.

Oslo is the capital and most populous city of Norway. It constitutes both a county and a municipality. The municipality of Oslo had a population of 709,037 in 2022, while the city's greater urban area had a population of 1,064,235 in 2022, and the metropolitan area had an estimated population of 1,546,706 in 2021.