Oscarsborg Fortress was built from 1846 to 1855 on two islets, with main batteries on Håøya. It took its name by royal resolution on August 23, 1855, after a visit by King Oscar I. Its three main guns were Krupp guns of 11 inches (28 centimeters), supplied by the German Empire.
An underwater barrier was built from 1874 to 1879. A torpedo battery inside a cave on Nordre Kaholmen entered service on July 15, 1901, and could fire six torpedoes without reloading. On April 9, 1940, the fortress engaged the German invasion fleet and sank the heavy cruiser Blücher. It surrendered the next day.
The torpedo battery remained secretly active until January 1, 1993. The officer training school closed in 2002, and in 2003 the fortress became a museum and resort. It was listed as a protected site in 2014.
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