It seems to me the Berlin Flak towers may be the last above ground fortresses of their size and magnitude ever built. However, like many dinosaurs of this period such as Battleships, surely it placed an unreasonable toll on Germany's industrial capacity and cried out like a sore thumb to be destroyed. Why weren't they? Why did these guns need to be elevated rather than distributed in parks and protected with sandbags? Did Berlin not have the space like in London? It would have plenty eventually!

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. These towers were each supported by a radar installation that had a retractable radar dish (the dish would be retracted behind a thick concrete and steel dome in order to prevent damage in an air raid). 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. The priority of the project was evidenced in the fact that the German national rail schedule was altered in order to facilitate the shipment of the necessary materials, namely concrete, steel and lumber to the construction sites.[citation needed]
With concrete walls up to 3.5 metres thick, Flak towers were considered to be invulnerable to attack with the usual ordinance 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 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 14km in a full 360-degree field of fire. The 3 flak towers around the outskirts of Berlin created a triangle of formidable anti-aircraft fire that covered the center of Berlin.

http://en.wikipedia.org/wiki/Flak_tower

The Flak towers described were as much a symbol of the power of the Reich as they were a means of fighting the bombers. The towers were meant to be the lynchpins of the anti-air defences, not the whole answer. the problems arose from the fact that the towers were poorly sited; that they were designed to defeat the smaller raids of 1940/41; and the German radar always lagged behind that of the Allies.
As a morale building exercise it was initially quite successful- but the major raids after 1943 led many Berliners to see them as a symbol of the declining state of the Reich. By then they had become little more than well armed bomb shelters. At the fall of Berlin in 1945 the towers housed (according to various sources) between 2000 and 5000 refugees each.

Man I would hate to have lived about 10km outside Berlin, with all that lead going up it would have to come down some where.