Signature of the city burnt into Manhattan's memory

Not long after a bomb ripped through the World Trade Centre in 1993, a video show was installed to mark the triumphal reopening in May that year. Footage depicted the downtown Manhattan skyline before and after the construction of the mighty, cloned buildings, with a commentary by Guy Tozzoli, chief executive of the World Trade Centres Association. Tozzoli told the story of how he led his staff down 77 storeys of smoking stairwell to safety. He remembered how the panic rose with the fumes: 'I built these towers,' he told them. 'I know they won't fall down.'

Now, people gather across the great avenues of New York, bereft of traffic, and stare into the empty sky at something they can see, but is no longer there.

Indeed, the absence of the twin towers makes their image even more vivid than their former omnipresence. Now, New York is wondering what to put there next: Mayor Rudolph Giuliani promises something 'even more beautiful'. Most on the street say the towers should be replicated exactly, by way of defiance. Others disagree: the empty space they leave should be no more than the site for a solemn memorial .

The World Trade Centre had come to supplant even the Empire State Building as the city's signature. It was New York's present, not its past. The completion of the towers was a recent chapter in the story of mankind's most audacious urban undertaking - Manhattan - which most of the citizenry could summon with its own memory rather than from city lore.

It was a picture-postcard emblem of the transition from stagnation in the Sixties, through a scrape with bankruptcy in the Seventies to the boom of the Eighties. Most people can recall a time when the skyline looked, well, as it does now. The towers became a way by which people told the time - according to which of their clean, vertical steel girders trapped the brilliant sunlight: a deep gold on the eastern edge during early morning, becoming paler towards midday, and deepening again towards dusk. At twilight, the towers shimmered; at night, they were like pearly towers. They had a spectral, quicksilver quality after rain, and if low clouds overlaid the city, they pierced them from below - an image immortalised by the cover of that great New York novel, Don DeLillo's Underworld .

When New Yorkers crossed the avenues cutting Manhattan north to south to buy coffee and papers or catch a subway in the morning, they usually threw the towers at least a glance - sometimes a lingering stare - out of respect or just aesthetic pleasure (including myself, every single day). The towers were invigorating, defiant, almost arrogant gestures by mankind pushing skywards into the blue.

Most important, the World Trade Centre was a city within a city, a frantic, workaholic community of 50,000 - half the population of Oxford - levelled into the dust of its own stone. That was the logic behind the rhetoric about an 'act of war' - not against an army, but caretakers,, secretaries, insurance consultants, messengers, stockbrokers, shoeshine boys, elevator and air-conditioning engineers, inside-traders, cappuccino and sandwich-makers - 110 storeys of social geology.

The biggest tenants were the Morgan Stanley banking behemoth, with more than a million square feet of office space, and the Bloomberg organisation, built by a man currently hoping to become mayor of New York.

But most people worked for smaller, humbler enterprises, giving it their best. Pamela Mills was a secretary for the Regus Business Centre on the 93rd storey of the second, southernmost, tower. After it collapsed she was in tears because she was still alive. Her sixth sense, however, told her that few other colleagues were. 'They called me last night and said to come in an hour later. That's why I'm here. I was just going into the building; that was before the tower came down. My traders and friends were still up there, on the 93rd storey, and I don't suppose I'll ever see them again.'

Olga Toussaint served iced latte, frapuccino and low-fat muffins on the mezzanine plaza. She knew many of the customers that came for their shot each morning: 'Some were very polite and stopped to say "Hi" each day. Some tipped a dollar and I made sure to have their drink started before they asked me for it. It was a nice place to work; I can't believe those people [who] I know how they like their coffee are under all that.'

There was more than finance in the World Trade Centre - it was also the headquarters of the US Customs Service, where Alan Barry worked. 'I was called at home and they asked: "Barry? Are you still alive?" We were all called at home. "Reckon I must be," I said. But I don't think everyone was in to take that call.'

Not every storey of the towers was let: as the Ides of Recession crept across Manhattan, so more office space was left vacant. But there is no such thing as unoccupied space in the urban termites' nest of Manhattan, and each night security guards spent more and more time looking for squatters and crack heads camped down in rooms that bond dealers had left behind.

The World Trade Centre had controversial origins, reaching back to a bitter battle over its construction. The towers rose from the ruins of the old financial district, sacrificed to make way for them. The man whose mission it was to build the centre was an Olympian bureaucrat called Austin Tobin, chief of the Port Authority of New York and New Jersey - a body accountable to no one, which wrested control of the project from both the city and state of New York. Ranged behind him were the combined muscle of political eminences, Wall Street, construction unions and hard cash.

The plan was to turn downtown Manhattan into the most valuable square mile of real estate on the planet. For his choice of architect, Tobin chose one of Mies van der Rohe's lesser-known disciples - Minoru Yamasaki, a son of Japanese immigrants. 'What I decided to do,' said Yamasaki, 'was the beautiful thing. Beauty through structure and technology, because that's our culture.'

A lecturer at New York University, Eric Darton, wrote an intriguing 'biography' of the World Trade Centre called Divided We Stand . It is a ruthless critique of the manipulative politics behind the construction of the towers, of Tobin and Yamasaki, but contains this observation on the aftermath of the 1993 bomb: 'The intention was to blow out the supporting columns along one wall of tower two, causing it to keel over and topple against its twin.' But, he writes, 'Yamasaki had engineered his towers to withstand the force of a 747 shearing into them - the nightmare scenario of an earlier "innocent" era.'