Don't miss the imposing Oscarsborg Fortress, which lies on an offshore island and dates back to the 1600s. It was the Oscarsborg batteries that sank the German warship Blücher on 9 April 1940, an act that saved the king and the Norwegian government from being captured, and which is dramatically depicted in the film The King's Choice. Ferries to the island depart every five minutes or so from Sundbrygga on the harbour in summer and less frequently the rest of the year.
There is even a hotel, a spa and a restaurant on the island if you want to extend your stay, and open-air concerts, including entire operas, are held here during the summer.
