Odderoya
When the Germans attacked Norway in april 1940, the norwegian coastal defense on Odderoya shelled the german invasion force. But the Germans answered the shelling and eight Norwegian defenders were killed. The Norwegian defenders still managed to sink the German freighter MS Seattle. After the fighting, the fort was taken over by the Germans who moved the cannons to other coastal defences.
Current status: Partly preserved/demolished with monuments (2017).
Location: 58°07'58.57"N 08°00'20.19"E
Get there: Walk from central Kristiansand.
Follow up in books: Greene, Jack, Massignani, Alessandro: Hitler Strikes North: The Nazi Invasion of Norway & Denmark, April 9, 1940 (2013).
Odderoya is a walking (recreation) area, but the traces of the war (including First World War) can be found everywhere for those looking. There are both monuments and information boards that remember and tells about the coastal defence during both world wars.