Flak towers were large anti-aircraft gun blockhouses used by the Luftwaffe to prevent overflights of key areas in certain cities in World War II. They also served as air-raid shelters for tens of thousands of people and to coordinate air defence. With concrete walls up to 3.5 metres thick, these 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 bombs which successfully penetrated much thicker reinforced concrete. Aircraft generally appeared to have avoided the flak towers. The Soviets, in assaulting Berlin, found it hard to make an impression on the Flak towers, even with some of the largest Soviet assault guns, the 203 mm howitzers. Soviet forces generally maneuvered around them, and eventually sent in envoys to seek their submission. Unlike the rest of Berlin, the towers tended to be better stocked with ammunition, and used their anti-aircraft 128 mm cannons to attack ground units. The Zoo Tower was one of the last points of defense, with German armoured units rallying near it at Tiergarten, before trying to break out of the encircling Soviet Red Army. The towers, during the fall of Berlin, formed their own communities. Being some of the safest places in the fought-over city, they were usually crammed with civilians, eventually forcing them to capitulate as supplies ran out. Each Flak tower complex consisted of a G-Tower (Gun Tower), and an L-Tower (Fire Control Tower). It appears the necessity of a separate radio tower was due to the massive shock waves created by the simultaneous firing of multitudinous 128mm Flak cannon in the gun tower which either interfered with the radio waves themselves used by the radio tower to locate enemy planes or with the ability of personnel to use the radio equipment.

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