Oscarsborg is a historic fortress located on an island in the Oslofjord, Norway. Built in the mid-19th century, it was designed to protect the capital, Oslo, from naval attacks. The fortress played a significant role during World War II, particularly during the German invasion of Norway in 1940. Today, Oscarsborg is a popular tourist destination, offering guided tours and beautiful views of the surrounding waters. Visitors can explore its well-preserved buildings, including the Oscarsborg Fortress Museum, which showcases the fortress's history and military significance. The island is also a serene spot for picnics and outdoor activities.