The structure of Bern's city centre is largely medieval and has been recognised by UNESCO as a Cultural World Heritage Site. Perhaps its most famous sight is the Zytglogge (Bernese German for "Time Bell"), an elaborate medieval clock tower with moving puppets. It also has an impressive 15th century Gothic cathedral, the Münster, and a 15th-century town hall. Thanks to 6 kilometres (4 miles) of arcades, the old town boasts one of the longest covered shopping promenades in Europe.

The structure of Bern's city centre is mainly what type of buildings?