Flak towers (German: Flaktürme) 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 m 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 Russians, in assaulting Berlin, found it hard to make an impression on the Flak towers, even with some of the largest Russian assault guns, the 203 mm howitzers. Russian 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 Russian 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.
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.