Akershus Fortress is a medieval castle and fortress overlooking Oslo's harbor, with construction beginning in 1299 under King Håkon V. It withstood numerous sieges over the centuries and was never taken. The castle was modernized into a Renaissance palace in the 17th century and remains an active military base. Highlights include the Royal Castle Chapel, the Norwegian Resistance Museum, and the Armed Forces Museum. Visitors can explore the grounds for free and enjoy stunning views over the fjord and city.