Teufelsberg was occupied by the Americans and the British during the Cold War and served as a listening station. Consider this hill and these abandoned structures one giant hearing aid. It was used for listening to Soviet, East German, and other Warsaw Pact nations’ military traffic.
Abandoned after the Cold War and left in shambles, it's now one of the many interesting abandoned places around Berlin to tour and learn more about the Cold War. In addition, the hill that the old spy station is built upon offers up some spectacular views of Berlin and its wooded areas.
It's "open" year-round, and the day I went it was blustery cold, so be sure to take warm clothes if going in the winter, as there are no heated structures.
You can tour and photograph the abandoned windowless structures on the weekends for a cost of US$20 and hear all of the spy stories to get your imagination running wild.
Tours are possible every Saturday and Sunday at 12:30 pm. If there aren’t enough English guests, the guide speaks German but may answer your questions in English between the stops.
More information: http://www.ottsworld.com/blogs/berlin-cold-war/