VIENNA -- Guidebooks do not mention them and many Viennese do their best to ignore them. Tourists search in vain for plaques explaining their history. But neither dynamite nor wishful thinking can rid the Austrian capital of six wartime monoliths casting dark shadows over its more illustrious charms.

Designed to defend Vienna against Allied bombings, the six concrete monsters are antiaircraft towers whose prominent placement was overseen by Adolf Hitler himself. Like other grandiose architectural schemes of the Third Reich, these citadels were built to last for centuries.

With steel-reinforced walls up to 10 feet thick and proportions rivaling the Arc de Triomphe in Paris, they loom on as Hitler prophesied. However, the defeat of Nazism rendered them unwelcome reminders of a brutal past. Foiled in the process were plans to clad the hulks in black marble and turn them into monuments to heroes of the glorious German victory.

Firepower from the towers' top platforms, perched as high as 200 feet above street level, proved incapable of thwarting the bombs that reduced many buildings to rubble and killed 8,770 civilians in the battle to end totalitarian terror. While the resulting debris has long been cleared away, the elephantine structures remain little changed. Their soaring gray silhouettes easily rattle the unaccustomed passerby who comes upon them as if encountering a latter-day variant of the Egyptian pyramids imposed upon Baroque and fin-de-siecle surroundings.

Since 1945, the Austrian government has considered more than a dozen plans to convert the windowless towers for civilian use. The artist Christo once wanted to wrap them in fabric, and painter Friedensreich Hundertwasser would like to decorate them. Several architects have proposed covering their drab exteriors with luxury flats and shops, and using their vast interiors for parking garages, theaters, hospitals or fitness centers. None of these projects has materialized.

The gradual transfer of the towers' custody from the federal government to the city of Vienna, described by officials as an administrative housekeeping measure, is now stirring a debate about their future. Fresh proposals to construct a restaurant and coffeehouse atop one of the towers and a public swimming pool above another are on the boards.

Obsessed with the contemporary significance of these unsettling architectural forms, a young Austrian filmmaker has produced a documentary to promote a more open public discussion about the towers' origins.

"One goes through the city and suddenly comes upon these towers and asks, 'What is this?' " Fridolin Schoenwiese says in an interview. "No one knows. They are truly a mystery. There is always talk of getting rid of them, but never is it said, 'What are these? What were these? Who built them? What did we think when they were built?' There has never been a real discussion."

Schoenwiese argues that the lack of such a discussion about one of the most visible traces of the war is symptomatic of Austria's reluctance to fully confront its Nazi past. "The young do not know what they are and ask about them," says retired psychologist Elfriede Hansen as she walks her dog around the tower dominating Vienna's Esterhazy Park, a 10-minute stroll from the city center.

Constructed between 1943 and 1944 by forced laborers, the towers form a strategic triangle around the heart of old Vienna's First District. "The Fuehrer considers comprehensive, urgent protection necessary for the city center of Vienna, which he regards as one of the most precious in Germany," reads a transcript of a 1942 meeting Hitler held with his chief architect and armaments minister, Albert Speer.

Friedrich Tamms, another German architect who had designed bridges along the Reichsautobahn, drew up plans for the Vienna towers to contain air raid shelters for a total of 40,000 civilians. Each was equipped with double-barreled cannon or flak guns, whose name stems from the German Fliegerabwehrkanone. They were completed just before the U.S. Air Force began attacks in April 1944.

Half of the six now stand empty and forlorn. One looming near the Ringstrasse boulevard behind the former stables of the Hapsburg emperors is used by the Austrian army as a high-security communications center and archive. Another in Arenberg Park serves as an army depot. The bottom portion of the Esterhazy Park tower is home to pythons and piranhas -- on display in an aquatic zoo located there since 1980.

Smaller towers were constructed in Berlin, Hamburg, Nuremberg and Munich. A postwar attempt to blow up one Berlin tower was only partially successful; the remains were covered with landfill and now form a hilly mound in a public park. The other German cities have since converted their towers for civilian use.

Austrian officials now say they want their more imposing structures to endure much as they are and bear witness to the Nazi past. Demolition would cost millions of dollars and could only be undertaken with great difficulty. This was highlighted in 1946, when children playing in the Augarten Park tower ignited some 2,000 pieces of munition left behind in its bowels. The massive explosion cracked an exterior wall but caused little other structural damage.

"The flak towers are part of our history, like the St. Stephen's Cathedral or the Hofburg," says Hannes Swoboda, the Vienna city council member responsible for urban planning. "I would oppose building over all of the flak towers, but all of them do not necessarily have to remain as they are. We have left traces of the Turkish era or Roman epoch, and for this reason I do not believe that they should all vanish or be converted."

City district superintendent Kurt Pint, who oversees the area including Esterhazy Park, hopes construction of a restaurant and coffeehouse atop that tower will get underway before the end of the year. The lower level of the structure will be unchanged. "The character of the flak tower must be maintained as a memorial to this wretched past," Pint says.

A general scheme for the Esterhazy Park district has been drawn up by a 12-member architectural team, known as Rastlos and comprising professionals from Milan, Berlin, Bologna, Vienna and London. "We as designers, and the Viennese, have to try to look at the flak tower as a friendly giant. And to try to consider it in a positive light, as opposed to the negative effect it had and still has for the area and the city, due to its initial purpose," the team writes in its proposal. "This tower is a war building. Thus we have to give preference to everything around the tower which is of life or reminds us of life."

Council member Swoboda says Viennese authorities plan greater efforts to educate the public about the towers' origins. "The number of people who had a direct connection to the flak towers ... is becoming ever smaller. Therefore the need to explain them is growing."

Says filmmaker Schoenwiese, "These towers are somehow foreign elements, and we must finally be clear about what they were before integrating them into the cityscape."