Flak tower in Vienna

In Vienna again for work. It was  a hot afternoon and we'd finished for the day, so I hopped on Tram 31 to have a walk by the Danube.  Spring has already turned to Summer and a few people on the tram were off for picnics.  Then I looked out the window and saw this. I supposed it was a leftover from the second world war, and jumped off to have a look.  The size of this was astonishing, as was the height of the balconies. Built by the Luftwaffe during WW2, apparently there are a few of these of various designs dotted around Berlin, Hamburg and Vienna, although a bit of research afterwards suggest this one is the biggest.  They had anti aircraft guns on top and when the RAF came over, thousands of people could go and shelter inside them.  Built often by forced labour, millions of tonnes of concrete and bricks went into them. Attempts to knock them down with explosives of course would take out most of the surrounding areas as well.  Nowadays these huge buildings, with walls about 4m thick, stand looming and silent, in many cases surrounded by trees and people sunbathing and playing frisbee.