From
http://www.olavsrosa.no:
The Coast Guard fortress Oscarsborg was built between 1845-1853. At the time, it was considered one of Northern Europe's strongest fortresses with 73 "full-sized cannons". The Fortress is most famous for the shots that sank the German warship Blücher early in the morning on April 9, 1940.
http://www.maritimequest.com/warship_directory/germany/pages/cruisers/blucher.htm