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. 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.
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. 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.
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 keeps 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
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.