Flak tower

Flak towers (German: Flaktürme) were large, above-ground, anti-aircraft gun blockhouse towers constructed by Nazi Germany. There were a total of 8 flak tower complexes in the cities of Berlin (three), Hamburg (two), and Vienna (three) 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 in France, and Heligoland in Germany.

The towers were operated by the Luftwaffe to defend against Allied strategic air raids against these cities during World War II. They also served as air-raid shelters for tens of thousands of local civilians.