Architecture: The last word in American accents: What can be done with an ugly old railway viaduct? And what about a seedy mainline station? Jonathan Glancey looks at London's answers

LUDGATE, the last massive architectural development London will see for some years, is a bravura four-acre, pounds 400m corridor of titanic office blocks, public spaces and public art that squats over the old London, Chatham and Dover Railway line, which once ran exposed for Londoners to see from Blackfriars to Holborn Viaduct. This was the line that from 1874 crossed over Ludgate Hill directly in front of St Paul's Cathedral. Today it is buried underground, courtesy of Ludgate's developers, Rosehaugh Stanhope Developments plc and the British Rail Property Board. This is also the last monument, for now, from the American architects to whom London turned in the late Eighties.

Ludgate, son of Broadgate (by the same development team) is not merely the end of an era - the exit of the American big boys from London - but a memorable work of civil and civic engineering. The architecture of its four grandiose office blocks (three by the American practice Skidmore, Owings & Merrill; one by the American-influenced British practice Renton, Howard, Wood Levin) is likely to provoke a mixed response. As if hedging its bets, Rosehaugh Stanhope has commissioned four impressively built and thoroughly serviced buildings in four different styles.

The best, because the most urbane, of this gang of four is 10 Ludgate Place (by SOM), a coal-black, ribbed and finned Gotham City block that snakes its way theatrically up and around Seacoal Lane. 1 Ludgate Place behind it (also by SOM) is another fine building: unlike nearly all the American-designed or inspired Post-Modern office blocks of the Thatcher boom, this one does not conceal its steel-framed structure behind ritzy polished marble cladding. It is designed to capture some of the spirit and energy of the huge civil engineering works essential to its construction.

If the buildings will be liked by some and hated by others, the scope and quality of the engineering achievement realised at Ludgate cannot fail to impress. To construct Ludgate, a busy commuter railway line had to be built over and the viaduct crossing in front of St Paul's removed without bringing the City of London to its corporate knees. The viaduct was removed and the railway at this point diverted in just 17 days in May 1990. This required 400 hours of continuous work.

Meanwhile, the developers paid pounds 3m for an archaeological dig on this historic site (unearthing such gems as a three-seater Roman lavatory: the River Fleet, which flows under the pavement here, was long used as a sewer). The new pounds 100m City Thameslink station, paid for by the developers, was designed by RHWL and opened in November 1991. A new public square - Fleet Place - at the heart of the development followed in June this year and the half-million square feet of offices are just nearing completion, on time.

Ludgate provides more than just architecture to argue over and engineering achievements worthy of Boy's Own; it also provides new pedestrian links for this revamped part of the City, Fleet Place (for music and other public events) and some of the most intelligently integrated public artwork of recent years.

More impressive even than new sculptures by Bruce McLean and Stephen Cox are the 18,000 tiles, hand-made and hand-fired by Rupert Spira, that face the 100 New Bridge Street building. Arranged as visual games for commuters striding along the side of this office block, they bridge the yawning chasm between traditional craft skills and the style and construction of an up-to-the-minute office block.

It is hard not to be impressed by the Ludgate development, even if the Post- Modern Classical block on Ludgate Hill is hateful. Well located and thoroughly equipped and serviced, the development is letting well during the slump, particularly to law firms. It may yet be extended (John Outram has designed an Assyrian-Egyptian style office block for a fifth site at 200 New Bridge Street), but it does mark the end of the adrenalin-fuelled American invasion.

Three years ago British architects were on the defensive, whimpering about the Americans who were threatening to destroy their livelihoods. Everywhere the beleaguered Brits turned, there was another fast-track, Post-Modern American office block looming above old city landmarks, nowhere more so than in London.

Skidmore, Owings & Merrill, the world's most muscular architectural practice (New York, Chicago, London), built the iron-pumping 14-storey building that forms the Bishopsgate front of Broadgate, the heroic office development at Liverpool Street; SOM also built massively at Canary Wharf, the New Lanark of the 20th century, capping the Isle of Dogs. Canary Wharf was crowned with an 800ft stainless steel tower designed by Cesar Pelli from New York.

Another American organisation, Kohn Pedersen Fox (KPF), which is so big that the design and construction of a few million square feet of high-quality, steel-framed, granite-clad offices can be left to the office junior, had also barged its way into Canary Wharf. The same team remodelled and pumped steroids and Post-Modern details into the old Art Deco Daily Telegraph building in Fleet Street. Meanwhile, Swanke Hayden Connell was issuing masterplans for the redevelopment of the former Spitalfields Market like so many papal bulls; it looked unstoppable.

There seemed little doubt in 1986-89 that only the Americans had the expertise, muscle and spunk to design new-age office blocks to astonish London. But the boom went bust: Swanke Hayden Connell did not get to rebuild Spitalfields, Canary Wharf fell off its Docklands perch and the Americans 'restructured' and otherwise 'slimmed down' their operations, and it was business (or lack of it) as usual for British architects by the end of 1990.

But the Americans will be back. They have, after all, been 'over here' several times before. And the buildings they have designed in the past, like Canary Wharf and 10 Ludgate Place today, are some of the most memorable, if not the most subtle. Who can forget Selfridges (Daniel Burnham with R F Atkinson, 1907-28); Bush House, home of the BBC World Service (Helmle & Corbett, 1925-35); or the Hoover Building on Western Avenue (Wallis, Gilbert & Partners, 1932-35)? And who could ever estimate the influence that American architects have had in Britain this century? The city centres of Britain are simultaneously enlivened and plagued with architectural Americana.

The completion next month of Ludgate will mark a lull in the American invasion. But the slightest hint of a boom and they will return with Rotring pens, T-squares and CAD programs blazing.

(Photographs omitted)