The guns that were on the steel reinforced concrete structures fell silent and were removed many years ago, but the buildings themselves were so strongly made that it was not easily possible to demolish them.
And so they remain, haunting the city’s skyline, where they became known as the Grey Elephants, the giant remnants of the Nazi’s last line of defence against Allied troops that still tower above Vienna’s streets.
The 170-foot-high concrete flak towers are now so much a part of the city that many admit they do not even notice them. Most are now as they were at the end of the war, empty and deserted.
But for the flak tower in the Mariahilferstrasse, it has been converted into a multipurpose structure that includes the huge aquarium, a climbing wall, a zoo and now a spectacular panorama restaurant.
The hammerhead shark tank containing 150,000 litres of water is undoubtedly one of the most spectacular attractions, but it also includes the overhead crocodile pool which puts visitors coming in through the entrance of the building underneath a crocodile enclosure.
In other parts of the flak tower are further aquariums. The crocodile pool is surrounded by a humid jungle fitted into a converted greenhouse on the side of the flak tower. And on the outside, climbers can be seen scaling to the top, peering in through the greenhouse windows at the exotic wildlife and plants inside.
The flak tower was built at the location because of the clear view it had of the sky but that now gives the new "Ocean Sky" restaurant an event location with an unrivalled view of the area as well. And of course to get to it, it involves passing by the aquariums and other exotic creatures kept in the former flak tower several levels below.
With a combination of natural attractions and now the restaurant at the top, the "House of The Sea" (Haus des Meeres) is a tourism magnet with over 570,000 visitors a year.
Manager Hans Koppen said: "Many of the guests that come to the restaurant or to events here have never visited our attraction before. It's a great combination and the great thing is we notice that they sometimes then come back with their families or friends."
Story courtesy of Central European News