Germany, being a country with so much history, has a lot of old and abandoned secret spots. This particular location that I found in Beelitz was a sanatorium until world war 1 when it was then used as a hospital for the Imperial German Army .
(color fact : Young Corporal Adolf Hitler was treated here following the Battle of the Somme)
After the splitting of Germany it was then used by the Soviet army as a hospital. Since 1995 the place has been abandoned and attracts homeless/artists. If you intend to go, be careful. The roofs, very much like Greece/EU, are on the verge of collapse.