Flak towers (German: Flaktürme) 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. They were used by the Luftwaffe to defend against Allied air raids on these cities during World War II. They also served as air-raid shelters for tens of thousands of people and to coordinate air defense.
With concrete walls up to 3.5 metres (11 ft) thick, flak towers were considered to be invulnerable to attack with the usual ordnance carried by Allied bombers. Aircraft generally appeared to have avoided the flak towers. The towers were able to sustain a rate of fire of 8000 rounds per minute from their multi-level guns, with a range of up to 14 kilometres (8.7 mi) in a 360-degree field of fire.