In the eye of the beholder

Lord Foster of Thames Side, as Britain's most celebrated architect this century insists upon being addressed, is changing the skyline of London more dramatically than any architect since Sir Christopher Wren. Just three years ago he had only two B-grade buildings in the capital.

Gemlike as it is, the Sackler Galleries at the Royal Academy are hardly high profile, while Foster Associates' office block topped with Norman's penthouse and own heli-pad in Battersea is a bit Blofeld. Lottery money changed all that. A terminus at the Dome, the first footbridge across the Thames in 100 years, linking St Paul's with the new Tate gallery at Bankside, the Mayor's assembly adjacent to the Tower of London, and now Wembley Stadium - Norman Foster's legacy will stand on the most historic sites in London. For how long?

In the footprint of every visionary architect is the demolition ball and chain wielded by another. When Wren built the majestic St Paul's he imagined that it would last for ever, and nearly 300 years later his masterpiece still stands. This century's place of mass worship is the football stadium, standing on hallowed ground and playing to bigger audiences. The footballer Pele even called the world-famous Wembley Stadium "the church of football". When Wembley was built in 1923, it was engineered on such a colossal scale that its contractor, Robert McAlpine, compared it with the Colosseum in Rome, one of the wonders of the Ancient World. Yet it passed its sell- by date in just 75 years.

To crash land his spaceship of a new arena there, Foster quite ruthlessly plans to remove the 34ft-high towers that stand guard at the north entrance. Bolted on to the monumental halo that Foster has tossed over the Wembley pitch, they would be about as much use as stiletto heels on a pair of Nikes. Everything has to work hard on a Foster building. Those decorative twin towers just stand around not doing anything, which is why they have to go. This was not a popular decision. Without those twin towers, Wembley wouldn't be Wembley any more, would it? English Heritage even put a preservation order on it. But so convincing was Foster that they then retracted.

Rather than argue that a pair of dated towers that look as if they were moulded in a sandcastle bucket would ruin his spaceship, Foster persuaded us that they were made of such crumbly paper-thin mortar anchored in concrete that they could not be moved and preserved. "Nothing is impossible," he admitted. "You can land a man on the moon. But at a price. The quantity surveyor estimates pounds 20m - but I'd reckon on a lot more. " So the twin towers will bite the dust.

Now the northern entrance will have four colossal masts visible at night from the centre of London. In fact, they thrust so high above the 134metre diameter roof ring - known as the halo - that they need to get air traffic control clearance to get off the ground. The Civil Aviation Authority regulations specify a maximum of 137m, so Norman kept the four towers to that height on the plan and scale model for his presentation to the Secretary of State for Culture, Media and Sport, Chris Smith, and football executives.

However, to support the huge roof they are going to be as much as 20m taller, which makes them half the height of the Eiffel tower. It is ironic that Wembley was built on the site of Watkin's Folly, Britain's answer in 1896 to the Eiffel tower. It got no higher than 45m before being demolished in 1902 because it was unsafe. Without the masts, Foster's roof would need a forest of columns inside. Rather than let the vexed issue of their size hold up his final designs in presentation to Chris Smith, Foster left out the air-traffic control issue to explain that each tower is angled at 75 degrees so that a line taken from the top of each would meet in the middle of the pitch at kick off.

Expressing structure in such a lyrical way is what makes Foster's buildings popular. For a cool rationalist, he is quite poetic. Foster kicked off the presentation of his designs to sport journalists with a video of some of the great moments in Wembley's 75-year history: the inaugural Cup Final of 1923; the Olympics of 1948; Stanley Matthews' Cup Final of 1953; Bobby Charlton scoring, crowds cheering, and upbeat booming music. "The futuristic building needs to keep that atmosphere," said Foster. "You can't measure it. The roar, the quality of the turf related to the sunlight, the processional route steeped in tradition past the royal box." He took sound recordings of the crowds in the existing stadium, and promises that his new building will deliver that kind of experience under one big retractable roof. He then set about rubbishing the old Wembley: bad sight lines, cramped seating, poor facilities - you'd be amazed the games ever got off the ground. No wonder he does not go to football matches there, or indeed anywhere.

Norman then turned his attention to some of Wembley's rivals for the next Olympics bid by mentioning in passing what's wrong with the technology that turns football stadium into an athletics arena.

The Stade de France in Paris has retractable tracks moving backwards and forwards, but they prevent the upper tiers from seeing the action. In an obvious hint that they will bid for the next Olympics, Foster has risen to the challenge of what he calls "the once in a lifetime event" of athletics at Wembley with a track raised 6.5 metres above the pitch which will reduce the number of seats to 67,000. Organisers of the pounds 475m revamp say that it will cost a further pounds 140m to convert the stadium into an arena for the Olympics because 80,000 seats are required.

New stadiums demand new technologies to meet a variety of uses, while protecting the pitch. Behind its apparent simplicity, Foster's Wembley conceals technical intricacy. Like an iceberg, most of the building's function lies below the surface, including escape routes for stars. Now Michael Jackson won't have to make his getaway from Wembley in a helicopter, while his space helmeted stand-in pretends to be in a mock spaceship.

Wembley's walls have been silent witness to some truly strange events since it opened for the British Empire Exhibition with white silk billowing down cement walls to represent the White Cliffs of Dover, military tattoos and ice sculptures, to the Stones belting out "Honky Tonk Women" while colossal inflatables kicked out their legs high above the stadium. Foster's new Wembley goes with the flow, acknowledging the important role corporate entertaining makes in putting money back into grassroots football - his banqueting hall seats 2,000 and there's a five-star, 200-bed hotel on site.

Nick Raynsford, Minister for London, has already chosen his Mayoral HQ - if he wins. In a miserable, two-building jump off between Foster Associates and Will Alsop, backed by two rival property developers and endorsed by Raynsford,Foster won with this spherical glass searchlight beamed on the Tower of London. The architect's wish to make the business of governing London transparent to the public resulted in the Chamber facing north to cut overheating from the sun. The south side is cut away like a cheese-wedge to shadow itself

The first new bridge across the Thames for 100 years, the Millennium Footbridge, is 300m long and 4m wide and links St Paul's with the new Tate at Bankside. A blade of light runs continuously along its deck and it is supported by two slender piers. The bridge, designed by Foster with Anthony Caro and engineer Chris Wise, began as sugar-lumps balancing on the rim of a teacup with a spoon crossing the void. Just as bridges make connections, Foster bridged the gap between two opposing councils: the richest, the City of London, and the poorest, Southwark

Bearing a rather striking resemblance to an airship that has touched down in the city, nose first, the Swiss Reinsurance Company - a 40-storey, 179.8-metre tall tower - is diagonally braced and appears to have string wrapped around its face. High tech goes eco-chic by rotating each successive floor within the bracing, so that voids at the edge of each floor plate, continually spiralling up, let in light and draw in air through slits. Every sixth floor supports a garden that closes off these atria, controlling air-movement