The New City Architecture exhibition is showcasing the daring designs of some of London's new tall buildings. Emma Cullinan reports

St Helen's Place, in the heart of the City of London, is a picture-perfect Regency cul-de-sac. This would be an ideal setting for the opening of a Dickens' story. It's a serene place, entered through large gates, complete with blazer-and-cap-clad gateman.

Bursting up through the end of this 19th century scene is Foster's rocket-like Gherkin, that looks down, at an angle, on the classically proportioned, symmetrical street. At night the Gherkin, designed by architects Foster and Partners, has shiny red lights near its apex, giving it a truly Thunderbirds look. More properly known as 30 St Mary's Axe, it has cut a new route through the city's architecture.

This 40-storey building has broken the mould of straight-up, steel-framed towers in terms of shape and height - and others are set to follow, including Richard Rogers Partnership's triangular Leadenhall Building which has just gone in for planning.

The City of London is taking the design of its buildings seriously. The Lord Mayor of the City of London, Robert Finch, has established an exhibition of New City Architecture, displaying 21 architecturally striking buildings, selected by an international jury.

Some are proposed buildings, many are at the planning stage, and others are already built, such as Eric Parry Architects' Scottish Widows building with its facade of load-bearing prefabricated Portland stone piers, that enable floor to ceiling glazing (which is set back from the facade).

This building is a new take on concrete monoliths with featureless, repetitive windows dotted all over them. It speaks a similar language but its openings aren't all one on top of the other, instead they are slightly askew. A great idea, although the building still has a heaviness.

Also in the exhibition is the long glass Wagamama restaurant, that protrudes from a tower, designed by Fletcher Priest Architects. This is an aesthetically pleasing as well as a practical design - it alleviates the effect of down draughts on diners and passing pedestrians.

More climate control is on offer at the base of City Point, a 36-storey tower refurbished by architects Sheppard Robson with early input from Santiago Calitrava. This has been softened by sculptural edges, with a curve at its top and a 14-storey galleria protruding from the base. Wind-tunnel experiments were carried out to make sure that the building's shape would create a decent micro-climate at ground level.

Other ground-level considerations in new city buildings include the solid base of the predominantly glass One London Wall, by Foster and Partners, which links the transition from the ground level at Gresham Street, to the first-floor walkways on its Barbican side.

In the Plantation Place building, by Arup Associates, limestone fins project from the base to create a solidity, and the building steps back at the eaves to maintain the scale of the street. Currently on site, this building forms an H-shape at the top which should add interest to the skyline.

The contrast between Regency St Helen's Place and Foster's rocket is typical of this area filled with sombre old buildings befitting the Square Mile's banks, insurance companies and shipping brokers. The traditional buildings were later joined by failed-modern designs of dubious quality thrown up in the rush to create merely functional office buildings, plus some post-modern classical - or just plain pastiche - offices.

As the need for more office space arises there seems to be an attempt to learn from past mistakes and ensure that new buildings are well designed. Another departure is the breaking through of the height barrier.

Until the 1950s no one was to build above the top of St Paul's Cathedral, at around 300ft, but that was surpassed and now the buildings are aiming even higher. Renzo Piano's Glass Shard - which is slightly outside the Square Mile - is due to be around 333m tall, and Rogers' Leadenhall Building will reach 224.5m, just surpassing its 217m-high neighbour the Minerva Building, by Grimshaw Architects, which has received planning permission.

Like Sir Christopher Wren a tad before them, Richard Rogers and Norman Foster appear to have the city sewn up. Of the 21 buildings in the exhibition, Rogers is responsible for three of them while Foster has five, including his Millennium Bridge, which got off to a shaky start.

Wren's office was given the task of replacing more than 50 churches after the great fire of London, creating a skyscape of towers, domes and spires. Now that the huge number of surrounding buildings have reached the same height, they've been effectively buried and certain corporate clients are set to create a new skyscape from the towers that will rise above the existing buildings. That's a tall order.

As ever, it's up to the clients but it appears that the City of London has a good-design agenda. Even in New York, the Empire State Building and Chrysler were commercial enterprises - built to engender profit rather than to please tourists and design aficionados - but they show how it's possible to please a lot of the people all of the time.

The city's new buildings will generally be clustered together, as has proved successful in New York and Chicago. The odd tower, shoved randomly in a city, tends to lack drama and can look out of place. It might get away with it if it is very special but not if it's a Dublin Liberty Hall affair.

Clusters, such as Ballymun and the Sheffield towers, have a sort of eerie presence but are rather odd set against a rural backdrop - clusters of towers usually signify the hub of a metropolis.

In its 2000 report on high buildings in Dublin, architectural firm DEGW suggested that any tall buildings be at locations such as Heuston Station (where Paul Keogh Architects' 117m tower of apartments is proposed), Spencer Dock and the South Docks, with links to transport, where clusters of higher rise buildngs could be situated without compromising the inner-city skyline. In general, though, the report recommended that Dublin remain a low- to medium-rise city with buildings that are higher than 50m being the exception.

It seems that we eventually get used to certain buildings. Centre Point, by Richard Seifert, at the top of Tottenham Court Road in London, caused a huge row when it was built. Unlettable, and accused of ugliness, it was seen as a disaster, but now it has its name proudly emblazoned across its top and has achieved a certain softness in comparison to some of the rubbish that has been built since. The Trellick Tower flats by Erno Goldfinger, in London's Notting Hill, and Berthold Lubetkin's High Point flats in Highgate, have become the places to live. Although, along with good design, they do have location on their sides.

Walking around the City of London, there is a sense of buildings being slotted into available gaps. Alsop Architects' proposed scheme at Puddle Dock includes a glass structure that seems as if has been reversed into a gap between buildings. This is a case of relaxed density turning into high density as land becomes scarce.

Ireland is facing this prospect. We've built our towns and cities outwards but are now having to do some filling in on brownfield sites, and are contemplating heading skywards. Hopefully we won't be reaching the giddy heights of larger cities - it's all a matter of scale. In a small town a five-storey building would achieve tower status, and even the O'Connell Street Spike plays on its height.

As the status-conscious grand men of San Gimignano in Tuscany realised, high towers can bring fame. The New City Architecture exhibition shows that whatever's going to be staring down at us, from above existing buildings, had better be striking and well situated.

Not every building in the City of London is good and even those designed by the skilled and the famous aren't liked by everyone - but design excellence is worth aiming for.

NewCity Architecture, Finsbury Avenue Square, Broadgate, London, runs until July 2nd. Admission free.