Bern's UNESCO-listed medieval Old Town is itself the main attraction — 6 km of arcaded sandstone walkways, the 15th-century Zytglogge clock tower, the Gothic Minster cathedral, and the medieval Bear Park on the banks of the Aare. The Zentrum Paul Klee, designed by Renzo Piano, holds the world's largest collection of works by the Swiss-German painter. The Einstein Haus on Kramgasse commemorates the years when Albert Einstein lived here and developed the Special Theory of Relativity.