After the Royal Air Force bombed Berlin on October 1940, Adolf Hitler ordered the construction of several flak towers to protect his cities against the allied air threat. All of them were designed by German architect Friedrich Tamms, employing hundreds of forced laborers and war-prisoners from all over Europe.
With concrete walls up to 3.5 meters thick, flak towers were considered to be invulnerable to attack with conventional bombs carried by Allied bombers. The towers, during the fall of Berlin, formed their own communities, with up to 30,000 or more Berliners taking refuge in a single tower during the battle. These towers, much like the keeps of medieval castles, were some of the safest places in a fought-over city, and so the flak towers were some of the last places to surrender to USSR forces, eventually being forced to capitulate as supplies dwindled.