With its twin towers surrounded by a black grid of scaffolding, the Our Lady Before Tyn Church looms over the Old Town Square here like the emblem of a crumbling city.

Everything in this once-spectacular 14th century monument seems to have rotted. Its thick slate and sandstone walls are porous. Its massive wooden roof beams are splintered and shredded. Intricately carved Gothic balustrades and gargoyles have reverted to misshapen lumps of rock.

This is the kind of slow destruction that threatens to gut a city whose architectural treasures survived half a dozen wars. And that is why Vladimir Kansky, the overseer of the Tyn Church's 30,000-square-foot sheath of planks and steel tubes, has invested a lot of hope in the painstaking work it disguises.

Hundreds of feet above the church's base, Kansky's masons are slowly chipping out rotted blocks of facade and blackened pieces of sculpture one at a time. In a workshop below, freshly quarried slate is being shaped with wooden mallets and spikes into exact copies of the ruined stone.

Behind its gloomy black curtain, the Tyn Church is quietly undergoing a stone-by-stone renaissance.

"It's exactly the same work as 600 years ago," said Kansky, whose construction company has spent 11 years on the project. "It's all manual work, with the same old tools. And it comes with a 300-year guarantee."

If Kansky is right, his laborious project may be a milestone in what has become an uphill battle to preserve this city's often stunning beauty from pollution, misuse and decades of neglect.

Prague, with its 4,500-acre Old Town laid out on the banks of the Vltava River since the 13th century, holds an array of churches, synagogues, castles and palaces virtually unmatched since two world wars ravaged other European capitals.

Yet if few other cities can equal such architectural riches, perhaps no other in Central Europe is as precariously propped up as Prague. Nearly every street here is at least partly obscured by the planks and pipes of scaffolding, and many of the city's best known monuments have been boarded up for years.

Some of the scaffolding, like that of the Tyn Church and town hall on the Old Town Square, or at the old Jewish Quarter home of novelist Franz Kafka, supports slow but steady renovation work. But the scaffolds on many other buildings serve simply to prevent a complete collapse or the raining of loose bricks on passers-by.

"The scaffolding is the result of a long history of neglect," said Josef Mayer, an art historian who heads a Prague city agency charged with preserving 2,000 historic monuments and buildings. "The preservation of buildings was practically stopped before the Second World War. Buildings got older and older, and no repairs were done."

Decades went by before Czechoslovakia's Communist authorities finally resumed the work of preservation. By then, dozens of the designated historic sites as well as hundreds of other buildings were tottering under a host of maladies.

The stone corrosion on the facade of the Tyn Church was only the beginning of Mayer's problems. In other churches, heavy stone arches and ceilings were found to be gradually pressing vertical walls apart, threatening collapse. Deep stone basements in more than 100 buildings first constructed in Romanesque style were producing humidity that was cracking walls and ceilings.

In the Convent of the Blessed Agnes, carved stone columns dating from 1234 not only leaned, "but sometimes the same column leaned in more than one direction," Mayer said. Reconstruction of the convent began 20 years ago and is still not complete.

With an annual budget of less than 300 million Czechoslovak crowns, or about $27 million, Mayer's office and a handful of specialized construction companies are slowly seeking to reverse the damage. But progress often comes with agonizing slowness, not only because of the limited budget but because of the delicate nature of the work.

"It's all manual work. Nothing can be done by machine," said Kansky, a construction manager with the Prague Construction Enterprise. "It's not the kind of work that an ordinary worker can do. He has to know his geometry and have a sense of three dimensions. Twenty years ago there were masons who knew the art, but the craft is dying out."

The restoration of the Tyn Church helped spark a small revival of masonry crafts in Prague as workers were trained to duplicate the monument's delicate carving. This work force, in turn, has become part of a small industry of experts, bureaucrats and craftsmen that has grown up around the project over more than a decade.

Before work on the church began, a commission of masons, reconstruction specialists, art historians and government officials traversed the scaffolding to test, measure, examine and sometimes debate every stone, beam and carving on the 264-foot-high structure.

The result was a plan detailing exactly what is to be replaced, what left in place and what falls in between.

"Sometimes you'll have a piece where the top is okay and the base is okay, but the middle is rotten," said Kansky. "So you have to cut out just the middle and copy it, then fit it back in with the original pieces. It's like putting together a Rubik's Cube."

In the upper parts of the church's two towers, up to 90 percent of the surface stones were marked for removal. Each one is numbered and measured, then used to make a metal mold.

During long winter months in the workshop at the base of the scaffold, the masons fit the metal molds to blocks of new slate and slowly chisel out exact copies of the old stones. Even the fastest worker, Kansky said, can take up to five weeks to produce one piece of a carved balustrade, measuring about three feet square.

Such exacting procedures mean that the Tyn Church's scaffolding, in place since 1977, may not come down until well into the 1990s. Kansky and his 40 specialized masons will spend the better part of their working careers renovating just one of Prague's historic buildings.

Yet this prospect is somehow satisfying.

"What can I say? I'm a born Praguer, and I love its monuments," Kansky said. "I only hope I live so long as to see this one finished. If the stone lasts 250 years, then we will have done well enough. Then somebody else will probably be here to do it again."