Oscarsborg Fortress is a coastal fortress located on the island of Oscarsborg in the Oslofjord, Norway. Built in the mid-19th century, it was designed to protect the capital, Oslo, from naval attacks. The fortress features a series of fortifications, including cannons and bunkers, and played a significant role during World War II. In April 1940, Oscarsborg Fortress famously sank the German cruiser Blücher during the German invasion of Norway. This event delayed the German advance and allowed Norwegian forces to regroup. Today, the fortress is a historical site and museum, attracting visitors interested in Norway's military history.