The Flak Towers (Flaktürme) remind people of WWII and look kind of out of place in modern European cities. These large concrete fortresses used to contain anti-aircraft guns to shoot down planes. Eight complexes of blockhouse towers were constructed by Hitler's government in Berlin, Hamburg, and Vienna. They were used by the Luftwaffe to block allied air raids and also served as air-raid shelters for locals.
Each tower consisted of two towers that were able to fire 8,000 rounds per minute with a range of about 14 km and could shelter up to 10,000 people.
While most of them were destroyed during and after the war, Vienna still has six flak towers in Arenbergpark, Augarten Park, and Stiftskaserne and Esterhazypark.
One of them now belongs to a military base of the Austrian Army and another one became an aquarium in 1956, called Haus des Meeres. In Arenbergpark, two gigantic flakturms tower over the trees: the Flakturm VIII G-Tower hosts the MAK Collection of Contemporary Art and the L-Tower (Leitturm or Lead Tower) is planned to be used as a data centre for an IT firm. Some of the abandoned towers serve as homes to colonies of pigeons, while locals discuss possible projects for them.