With concrete walls up to 3.5 metres thick, Flak towers were considered to be invulnerable to attack with the usual ordinance carried by Allied bombers, though it is unlikely that they would have withstood Grand Slam bombs which 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 14km in a full 360-degree field of fire. The 3 flak towers around the outskirts of Berlin created a triangle of formidable anti-aircraft fire that covered the center of Berlin.[citation needed]
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 were some of the safest places in the fought-over city and some of the last places to surrender to Allied forces, eventually forced to capitulate as supplies ran out.
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.[citation needed]
墙厚是3.5米,还不止3米。