During a recent visit to the Augarten in the 2nd District, I snapped a few pictures of one of Vienna’s Flak Towers.
There are a number of these towers scattered across the city. Built during the last war, they were designed to provide Vienna with an air defence system. Another of these towers is used accommodate an aquarium and zoo called the ‘Haus des Meeres’ (House of the Sea). http://www.haus-des-meeres.at/de/Zootiere.html
The Red Army tried to demolish the towers, by blowing them up, during the Allies occupation of Vienna but the attempt left surrounding buildings damaged and other formers of demolition were considered too costly.