In Vienna there are six enormous flak towers, built by the Nazis with forced labour between 1942 and 1945, the purpose of which was to serve as anti-aircraft defences. Luckily, the Second World War ended before they came into use for anything other than shelters. Hamburg and Berlin also had such towers.
The towers were built in pairs from reinforced concrete with walls 8ft thick. The two towers that I photographed yesterday are 400 metres apart in the Augarten. This is a large park in the city with forested areas divided by tall hedges and gravel paths. One (the Feuerleitturm) is 172 ft high for radar, and the other (the Gefechtsturm) is 180 ft high for artillery. At the end of the war it was decided that these towers were indestructible, so they have remained in place. A few years later, an explosion in the Gefechtsturm raised the roof -- literally -- and it then fell back again, causing damage, which has been contained by means of wires stretched around the tower.
There are several pages about them on the internet (mostly in German), but these three (in English) are quite interesting and have additional photos: A : B : C
The Feuerleitturm (for radar)
The Gefechtsturm (for artillery)
David