Zitadelle Petersberg Erfurt

Situated on the Petersberg hill northwest of Domplatz, this 36-hectare citadel ranks among Europe’s largest and best-preserved baroque fortresses. While most interior buildings are closed to the public (and daubed with stencils by guerrilla artists), it sits above a honeycomb of tunnels that can be explored on two-hour guided tours (in German), run by the tourist office. Otherwise, it's free to roam the external grounds, and to enjoy fabulous views over Erfurt. Built on the site of a Benedictine monastery, the citadel numbers Napoleon among the many who have occupied and developed it over the centuries.