Flak towers (German: Flaktürme) were large, above-ground, anti-aircraft gun blockhouse towers constructed by Nazi Germany. "Flak" is short for anti-aircraft gun in German: Flugabwehrkanone. There were eight flak tower complexes in the cities of Berlin (three), Hamburg (two), and Vienna (three) from 1940. Other cities that used flak towers...
.JPG/500px-Wien_-_Flakturm_Augarten_(1).jpg)