|Flak towers (German: Flakturm) were large AA 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 3.5m thick these towers were considered to be invulnerable to attack with the usual ordnance carried by Allied bombers, however it is unlikely that they would have withstood Grand Slam bombs which successfully penetrated much thicker reinforced concrete .|
For the 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 they remain to this day.