Flak tower 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 ( Main ) . They were used by the Luftwaffe to defend against Allied air raid s on these cities during World War II . They also served as air - raid shelter s for tens of thousands of people and to coordinate air defence . History and uses . After the RAF ' s raid on Berlin in 1940 , Adolf Hitler ordered the construction of 3 massive flak towers to defend the capital from air attack . These towers were each supported by a radar installation that had a retractable radar dish ( the dish was retracted behind a thick concrete and steel dome in order to prevent damage in an air raid ) . The flak towers , the design of which Hitler took personal interest in and even made some sketches for , were constructed in a mere 6 months . The priority of the project was such that the German national rail schedule was altered to facilitate the shipment of concrete , steel and timber to the construction sites . With concrete walls up to 3 . 5 metres thick , flak towers were considered to be invulnerable to attack with the usual ordnance carried by Allied bombers , though it is unlikely that they would have withstood Grand Slam bomb s which , while not designed for this , had successfully penetrated much thicker reinforced concrete . 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 km in a full 360 - degree field of fire . However only the 128 mm guns had effective range to defend against the RAF heavy bombers . The three flak towers around the outskirts of Berlin created a triangle of formidable anti - aircraft fire that covered the centre of Berlin . The flak towers had also been designed with the idea of using the above - ground bunkers as a civilian shelter , with room for 10 , 000 civilians , and even a hospital ward , inside . 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 keep s 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 . The Soviets , in their assault on Berlin , found it difficult to inflict significant damage on the flak towers , even with some of the largest Soviet guns , such as the 203 mm howitzers . Soviet forces generally manoeuvered around the towers , and eventually sent in envoys to seek their submission . Unlike much of Berlin , the towers tended to be fully stocked with ammunition and supplies , and the gunners even used their anti - aircraft 20 mm cannons to defend against assault by ground units . The Zoo Tower was one of the last points of defence , with German armoured units rallying near it at Tiergarten , before trying to break out of the encircling Soviet Red Army . For a time after the war , the conversion to representative objects with decorated facades was planned . After the war was lost , the demolition of the towers was in most cases unfeasible and many remain to this day . Flak tower design iterations . Each flak tower complex consisted of : The evaluation of even larger Battery Towers was commissioned by Adolf Hitler . These would have been three times the size and firepower of flak towers . Towers . Flakturm I – Berliner Zoo , Berlin . " see main article " Zoo Tower Flakturm II – Friedrichshain , Berlin . Both towers were covered over and now appear to be natural hills in Volkspark Friedrichshain . The G - Tower , known as " Mont Klamott " ( Rubble Mountain ) in Berlin , was the inspiration for songs by singer - songwriter Wolf Biermann and the rock band Silly . This tower , containing six levels below the rooftop , includes in its design , as part of its air - raid shelter , two identical spaces for protection against gas attacks , one on the first floor ( above ground level ) and the other on the second floor . Both in Tower 1 , they are about 300 sq . m . ( 3 , 230 sq . ft . ) in area , and have six windows ( openings in the wall ) .