Akershus Fortress

Akershus Fortress or Akershus Castle is a medieval castle that was built to protect and provide a royal residence for Oslo, the capital of Norway. The castle has also been used as a military base, and is today a museum, open for public.

It is not known exactly when the construction of the castle started but it is believed that it took place around the late 1290s, by King Haakon V, replacing Tønsberg as one of the two most important Norwegian castles of the period . It was constructed in response to the Norwegian nobleman, Earl Alv Erlingsson of Sarpsborg’s earlier attack on Oslo that occurred in 1287. In the aftermath of the attack, it became clear that the city’s existing defences weren’t effective and therefore, a stronger defensive centre was needed.

Wikipedia