Oscarsborg Fortress was built on an island close to Drøbak at one of the narrowest points of the Oslo Fjord, a location chosen strategically to defend the capital when Norway
was under Danish rule. The site is best known for its role in World War II's Battle of Drøbak Sound. The upgraded horseshoe-shaped fortress prevented the Nazis from advancing to Oslo, and its coastal guns even sank the heavy cruiser Blücher
. Today the site is a popular tourist attraction, with boats running from Drøbak year round.