By Bethany Bell
BBC News, Vienna
The Leitturm in the Arenberg Park is one of the best-preserved towers
Scattered through Vienna are six huge anti-aircraft towers, a grim reminder of the city's Nazi past.
The flak towers were put up during the Second World War between 1942 and 1945. They were built in pairs in parks in the heart of the city.
Guns to shoot down Allied fighter planes were placed high on the concrete bastions and hundreds of civilians took shelter inside during bombing raids.
But the architectural historian, Ute Bauer, who is taking part in a study of the towers, says their main purpose was propaganda.
"The towers were meant as a sign of the military strength of the Third Reich, to show that the allied bombers could be defeated.
"In 1943 when the towers were built, the authorities already knew the bombers flew higher - so they were of no military use, but they built them regardless."
She says that underground air raid shelters were more effective protection for the civilian population than the landmark towers.
'Horror of war'
The flak towers were built by forced labour. On the walls inside you can still see graffiti, written in chalk and pencil, by prisoners of war.
"Milano, poi morire" wrote one Italian prisoner. "Milan, then die".
Today one of the flak towers houses an aquarium and another is used by the Austrian army. But there is controversy about what should be done with the others.
Getting rid of the reinforced concrete hulks would be too difficult and expensive, according to Sandro Forst of the Vienna City Administration.
"Even if you put a bomb inside you couldn't destroy the tower so you would need very strong diamond saws to cut it into pieces," he said.
"And to cut up a whole tower is, from a technical point of view, impossible."
The towers have few windows and their cold concrete halls and echoing stairwells recall a period in history that some Austrians would prefer to forget.
Calls by Vienna's MAK Museum for Applied Arts/Contemporary Art to turn one of the towers into a centre for modern art have so far failed to materialise.
A view inside one of the flak towers
But now the Vienna City Council is considering plans to turn one of the best-preserved towers, the Leitturm in the Arenberg Park, into a data storage centre.
Sandro Forst says upkeep costs are "rising enormously", and that rent from a commercial storage company would help with the maintenance of the listed buildings.
Above all, he says, a flak tower is secure: "The data centre must be kept safe from any kind of terrorism.
"The flak towers are made of very stable concrete, they are well located and they are very safe because of the concrete walls."
But Sabine Gretner from the Green Party believes the towers should be opened to the public - as a reminder of a dark period of history.
She says that because of the "Austrian political landscape, it is very important to know about the Second World War".
"We have got right-wing politicians, and young people are starting to vote for them. And these buildings are so strong inside that you can feel the horror of war. I want people to have this feeling so that it cannot happen again."
At the moment, there are not even signs explaining the history and function of the towers. But their looming presence above Vienna's parks and playgrounds cannot be ignored.