A jewel in the Drøbak narrow is definitely the Oscarsborg Fortress.
Originally the fortress was created to resist any attack from the sea but only on the 9th of April 1940, the Oscarsborg festning endured its first real trial.
Later, during the occupation of Norway, German forces were stationed right here at Oscarsborg.
Since there are several options on how to get to the island try to work one of those if you’re ever in a mood for discovering the Oslo surroundings.
On the weekends there’s a direct ferry from Akery brygge to the Fortress or you can take the bus from Oslo bus terminal to Drøbak and than use the small warship that goes from Drøbak to the south and north Kaholmene islands that hostess the famous Oscarsborg fortress.