H/T Beyond The Band Of Brothers. 

A look at German defences during World War II.

On August 25, 1940 the first RAF raid reached Berlin with the objective of bombingTemplehof Airport. The attack, an embarrassment to the Nazi regime, triggered Hitler into issuing a series of orders to strengthen the air defense of major German cities and personally drawing sketches for an anti-aircraft “fortress” with gun platforms raised above the neighboring buildings. Over the course of the next few years, eight Flakturms (flak towers”) were built in Berlin, Hamburg and Vienna with several others in the works and similar but smaller installations elsewhere.

The first towers, three in Berlin and one in Hamburg, were built in open park areas in order to maximize the range of their batteries. The three Berlin towers were arranged in a triangle around the city center, providing 360° cover in a radius of 8.5 miles. The building project was completed in only six months, with railway timetables being altered to allow the some 1,600 tons of building material to be delivered each day.

A flak tower consisted of two above-ground concrete bunkers: the G-tower (Gefechtsturm, “combat tower”), which held the main battery, and the L-tower (Leitturm, “lead tower”), which was responsible for fire control. The G-tower was 130ft high and 230ft to a side with bastions at each corner. The walls and roof were 10ft and 16 ft thick, respectively, and made of reinforced concrete. This massive size was necessary not only to protect the interior but also to support the weight and absorb the recoil of the 25-ton guns at the top.

One of the quad guns of the Berlin Zoo flak tower. The fire control L-tower can be seen in the background to the right
The standard armament for the G-tower consisted of 4 twin-barrel 128mm cannons, super-sized versions of the infamous 88mm Flak, and 8 four-barrel 20mm guns, with the smaller guns situated on protruding ledges below the roof. The inside of the tower housed five stories of ammunition rooms, crew quarters, hospital wards and air raid shelters with the capacity to protect 10,000 civilians. Artillery rounds were lifted to the roof by a mechanical elevator system.

The L-tower had the same height as the G-tower, but was comparatively more slender. The roof was equipped with searchlights and a radar dish that could be retracted into a shaft for protection. It also had 8 quad 20mm guns. The two towers, some 200 yards apart, were connected to each other with tunnels. Second and third generation flak tower complexes (one in Hamburg and two in Vienna) were built on a less grandiose scale, but using the same main armament.

Responding to an air raid alarm atop the a flak tower in 1944
The 128mm twin guns needed a crew of 21 men each. As the war dragged on, members of the flak crews grew increasingly younger. At the end of the war entire school classes of 14-16-year-old boys were conscripted as flak helpers, first to man the searchlights, later to handle the guns themselves.

In Berlin, the flak towers were the last pockets of resistance during the final days of the war. The victorious Soviet troops had to lay siege to them with tanks and artillery, suffering heavy losses from the rooftop guns but still unable to seriously damage the structures, eventually forcing the Russians to negotiate the towers’ surrender. By this time the towers were overcrowded with civilians three times their capacity and the air inside was hard to breathe.

Hitler Youth manning a gun on the Humboldthain tower in Berlin
After the war, attempts were made to demolish the towers. However, their thick concrete walls resisted explosions. Only the tower located in the Berlin Zoo was completely blown up and then buried. The other towers, or what was left of them, acquired new roles. Today, you can find nightclubs and music stores, an aquarium, an art storehouse and other public attractions in them. Two of them are even being considered for use as a solar power plant and a secure data center.
One of the Berlin towers during demolition attempts. The remains are now covered over and appear as a natural hill
The G-tower in Augarten, Vienna