The flak towers were a nightmare for allies in second world war

Flak towers were 8 complexes of large, above-ground, anti-aircraft gun blockhouse towers constructed in the cities of Berlin (3), Hamburg (2), and Vienna (3) from 1940 onwards. Other cities that used flak towers included Stuttgart and Frankfurt. Smaller single-purpose flak towers were built at key outlying German strongpoints, such as at Angers, France and Heligoland, Germany.