The medieval city was surrounded by a mighty wall, which was secured by 12 towers. Some of the towers of this city fortification are still quite well preserved, such as today's clock tower, the Schifferturm and the towers in Stephanienpark and in Fridrichsallee.
The four heavily fortified gates through which one entered the city are no longer preserved. The city park itself is an oasis of relaxation in the heart of the city.