Dating back to the beginning of the 14th century, only the rock foundations remained after it was destroyed by invading Swedish troops in 1621. It was rebuilt in 1650 with 2.5m thick walls to protect its valuable contents inside and was obviously successful, as nine Russian cannonballs that hardly made a dent were later embedded in its walls by cheeky masons. At the end of the 19th century a German student fraternity bought the building, but today it's part of the War Museum.