Just south of Oslo, the German navy began in 1940 to build a facility for its own personnel. The camp was built by Soviet and Norwegian prisoners of war and completed in 1942. It was contemporary of the time and consisted of barracks (two floors), administration, guard house, garage, workshops, casinos, shelters and parking space. 

Current status: Preserved (2023).

Location: 59°52' 49.13" N 10°46' 04.86" E

Get there: Car.

My comment:

The camp was not demolished after the war but was given other functions, both military and civilian functions. In 2018, the buildings were declared as cultural-historical important and therefore protected.

Follow up in books: Greene, Jack, Massignani, Alessandro: Hitler Strikes North: The Nazi Invasion of Norway & Denmark, April 9, 1940 (2013).