Debate about London's structure and infrastructure is currently concentrated on the high and the deep, skyscrapers and the Underground. The recent bombs brought to the surface the danger inherent in those 620 miles of dark, fetid tunnels while a slew of skyscrapers currently seeking planning permission looks set to transform the city's ragged skyline.

When terrorists attacked New York they naturally chose a skyscraper: the city has defined and expressed itself through its skyline for more than a century. In London they targeted the Underground, not because the city is short of theatrical buildings but because, more than anything, it is the Underground that presents the most recognisable and emotionally fragile symbol of the city. Escalating to the surface, we pass layers from Roman London, through the ashes of the medieval city and through the memories of a network that has been both target and shelter from bombs, a place of womb-like refuge, sweaty danger and increasing paranoia. It is the city's dark subconscious, in diametric contrast to the neutral, corporate blandness of the dumb glass offices dominating contemporary urban architecture.

The paradox is that, with all this concentration on scraping ground and earth, there is no serious examination of what is happening in between, on the surface. Nine enormous and hugely unsophisticated skyscrapers are being mooted by the world's architectural mega-corps, from the Bishopsgate Tower (1,020ft) and the Shard of Glass at London Bridge (1,000ft) to the Leadenhall and Minerva Towers in the City (both over 700ft). The aesthetic (if not commercial) success of Lord Foster's Swiss Re Tower (at only 590ft) has encouraged this surge of phallic architectural abandon and it is being stroked by, among others, the Commission for Architecture and the Built Environment, the deputy prime minister and the mayor.

I have no problem with skyscrapers, only with what happens at their feet. New York is an extremely dense city extruded from a grid. The setbacks that were introduced to ensure that at least some light reaches its canyon-like streets created the ziggurat form of the city's best buildings, but also allowed the structures to follow the street line so that the dense street network remains largely unbroken and, while the eyes of pedestrians may be sucked upwards towards impossibly tall pinnacles, their feet are dragged along bustling, coherent streets.

The London model dictates that, where a skyscraper is built, open space must be left around it, creating dim plazas. Consequently, tall buildings, while they do increase office space, fail to increase the density of the city, instead merely prodding the skyline with primitive architectural fingers, the sole aim of which is to create a recognisable logo. This leaves blank, unnecessary plazas, inevitably filled with the usual coffee shops and chain stores, the city becoming in effect suburbanised. More London, an otherwise well-intentioned riverside scheme in Southwark, presents the perfect example of this arid style of development, a scheme that would look comfortable in Toronto but has nothing to do with the complex grain of London.

New York, like Venice and Istanbul, is a city best viewed from the water, an extraordinary vision of what mankind can achieve, the apex of an urban civilisation that still appears futuristic. London, conversely, is still a city best understood from the pavement, the juxtapositions of age, material and scale, of tight lanes and leafy Georgian squares a constant delight. From Dubai to Shanghai, big cities are now attempting to define themselves through new skyscrapers, but most are cities trying to build themselves an identity from a standing start. The form of London was substantially defined at least 400 years ago and it is a city in which people, despite the bombings, already want to be. It does not need bland cliffs of glass affording views into all-but-identical corporate lobbies, but a series of intelligent interventions that acknowledge its layers of history and detritus, the memory of past incarnations and a recognition that so much of its form is tied up with what lies beneath.