Oscarsborg Fortress sits on small islands in the narrowest part of the Oslofjord, a few minutes by boat from Drøbak. The setting is peaceful now, with lawns, old batteries, harbour views, and museum displays, but the place carries one of Norway's sharpest wartime stories.
Give it time. The ferry crossing, the gun emplacements, the museum rooms, and the views back toward Drøbak make this more than a single monument. Even travellers with only a light interest in military history tend to remember Oscarsborg, because the geography makes the drama obvious.