Air defense towers used by Nazi Germany
Flak towers () were large, above-ground, anti-aircraft gun blockhouse towers constructed by Nazi Germany. There were a total of 8 flak tower complexes in the cities of Berlin (three), Hamburg (two), and Vienna (three) from 1940 onwards. Other cities that used flak towers included Stuttgart and Frankfurt. Smaller single-purpose flak towers were built at key outlying German strongpoints, such as at Angers in France, and Heligoland in Germany.
The towers were operated by the Luftwaffe to defend against Allied strategic air raids against these cities during World War II. They also served as air-raid shelters for tens of thousands of local civilians.
History and uses.
After the RAF's raid on Berlin in 1940, Adolf Hitler ordered the construction of three massive flak towers to defend the capital from air attack. Each tower had a radar installation with a radar dish which could be retracted behind a thick concrete and steel dome for protection.
Hitler was interested in the design of the towers, and even made some sketches. They were constructed in six months. The priority of the project was such that the German national rail schedule was altered to facilitate the shipment of concrete, steel and timber to the construction sites.
With concrete walls up to thick, their designers considered the towers to be invulnerable to attack by the standard ordnance carried by RAF heavy bombers at the time of their construction.
The towers were able to sustain a rate of fire of 8,000 rounds per minute from their multi-level guns (albeit mostly smaller-caliber shells, such as the 2cm FlaK 30), with a range of up to in a 360-degree field of fire. However, only the FlaK 40 guns had effective range to defend against the RAF and USAAF heavy bombers. The three flak towers around the outskirts of Berlin created a triangle of anti-aircraft fire that covered the centre of Berlin.
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 a hospital ward inside. During the Battle of Berlin, occupants formed their own communities, with up to 30,000 Berliners taking refuge in one 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 the Red Army, 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 M1931 howitzers. 
After the war, the demolition of the towers was often considered not feasible and many remain to this day, with some having been converted for alternative use.
Design iterations.
Each flak tower complex consisted of:
The G-Towers were square and tall, usually armed with eight (four twin) 12.8 cm FlaK 40 and numerous 37 mm Flak and 32 (eight quadruple) 20mm Flakvierling guns.
L-Towers were , usually armed with four quadruple 20 mm guns.
G-Towers were , usually armed with eight (four twin) 128 mm guns and sixteen (four quadruple) 20 mm guns.
L-Towers were , usually armed with forty (ten quadruple) 20 mm guns.
The G-Towers were , usually armed with eight (four twin) 128 mm guns and thirty-two (eight quadruple) 20 mm guns.
The evaluation of even larger Battery Towers was commissioned by Adolf Hitler. These would have been three times the size and firepower of flak towers.
Towers.
Flakturm I – Zoo Tower – Berliner Zoo, Berlin.
The tower built near the Berlin Zoo was the first-generation type and covered the government district. It was also used as a repository for artefacts from the Berlin Museum. The occupants surrendered to Soviets on 30 April 1945. In 1947 the British blew up the G-Tower on the second attempt with several tons of explosives. The L-Tower was demolished first in July.
Flakturm II – Friedrichshain, Berlin.
Both towers were covered over and now appear to be natural hills in Volkspark Friedrichshain. The G-Tower, known as "Mont Klamott" (Rubble Mountain) in Berlin, was the inspiration for songs by singer-songwriter Wolf Biermann and the rock band Silly.
Flakturm III – Humboldthain, Berlin.
The third of the first-generation flak towers were built at Humboldthain.
The G-Tower was partially demolished after the war; one side remains visible. The interior can be visited. . The L-Tower was partially demolished after the war; some walls remain visible. 
Flakturm IV – Heiligengeistfeld, Hamburg.
Heiligengeistfeld (1st Generation)
This tower contains six levels below the rooftop and includes in its design, as part of its air-raid shelter, two identical spaces for protection against gas attacks. One is located on the first floor (above ground level) and the other on the second floor. Both in Tower 1, they are about 300 sq. m. (3,230 sq. ft.) in area and have six windows (openings in the wall).
The L-Tower was demolished after the war . The G-Tower was transformed into a nightclub with a music school and music shops. In 2019 the NH Hotel Group announced plans to turn it into a luxury hotel with a rooftop garden.Construction was planned to take place in 2021 and opening in 2022. After the reconstruction the height was to increase to 58m with five additional floors. There were to be thirteen stairs.
The construction was completed 2 years behind schedule in 2024, The hotel, 'REVERB', first opened in April, with a capacity of 134 guest rooms.. The rooftop garden opened in July, and contains 4,700 trees and shrubs.
Flakturm VI – Wilhelmsburg, Hamburg.
The tower at Wilhelmsburg is a 2nd generation type.
The G-Tower remains to this day, , the L-Tower was demolished after the war.
References.
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