The walls surround the city and can be travelled by bike or on foot. In fact you will see a steady stream of runners and joggers. Here and there sections of the walls dip to the original tunnels and chambers used throughout the city's history. One of these sections even contained a small exhibit of statues and models depicting attractions within the city and how it changed over time.
The view is beautiful, you can see most of the inner city inside the walls and the mountains outside.
As a nice way to close or begin a trip to the city, I recommend the walk; it's even possible to rent a bike to get around that way.