Spandau Citadel is one of the most important and best-preserved Renaissance fortresses in Europe. In the 16th century, developments in weaponry rendered older castles useless. The Citadel is filled with sights that visitors can explore on their own. Anyone looking for a bit more should absolutely take part in a guided tour: a tour will allow you into otherwise closed-off areas such as the famous underground casemates, vaulted rooms and the powder magazine in Bastion Königin.