Vaguely related: The Sokos Hotel in Talinn, Estonia was one of the most luxurious properties ever developed by the Soviet Union. Why? With Estonia being one of the most western-most, westernized Soviet republics, the KGB figured it'd be a great place to snoop on Westerners (and people meeting with Westerners). Thus, the entire 23rd floor was a KGB listening post. When Estonia fell to democracy, the KGB literally got up and left, virtually as fast as they evacuated Chernobyl. The Estonians found cigarettes in ashtrays, sandwiches half-eaten, etc. The KGB straight BAILED. So the Estonians left everything exactly as it was and called it a museum. It's worth a Google Image Search.
Wow, never heard of that. Definitely seems like the kind of thing the KGB would do. Reminds me of these movies where a strange floor is discovered in between floors. Kinda "being john Malkovich" style. I didn't write out the story of the place in post because even if I kinda know the story, I honestly don't feel like doing research to get all my facts right. Some people have already written summaries about the history of the place: http://www.abandonedberlin.com/2010/06/teufelsberg-abandoned-spy-station.html
I focussed more on my personal experience of it, but the history of Teufelsberg is definitely fascinating and worth learning more about.