"All a planner can do is create mediocrity out of awfulness." That remark comes from Peter Wynne Rees, the City's cultured and thoughtful planning supremo - and he offers it with deliberately mordant irony, for he's at the architecturally-minded heart of changes that could, in the next decade, change the evolving face of the City for the better."Eighteen or 20 years ago," he notes, "there were perhaps only six business architects working in the Square Mile It was all rather boring. If it fails to interest the bulk of passers-by, what will be the point of it?Sir Nicholas Grimshaw faces a similar challenge at Houndsditch, where the proposed public domain around the Minerva tower is even bigger than the Leadenhall Street swath, and will require new traffic routing. The tower here - part of it, anyway - will give the City a svelte, gleaming version of New York's Flatiron Building. But, again, how will we experience it? Will it (pace the commentator Simon Jenkins) turn out to be a damnable architectural ghetto-blaster? Or might it create a compelling new place on mysterious old turf? And will it drive another stake into the heart of future developments of the cheapjack, classical-postmodern sort that have so poisoned the City's vistas for the best part of three decades?The questions come easily enough, but the solutions are tortuous. However, two much more important things are likely to be achieved: the new tower will not only oversail part of its "footprint" with a publicly-accessible caf?one, but the square will be subject to a complete makeover by the spatial consultants Lovejoy.If this development is to have the kind of 21st-century pulling-power required by City planners intent on maintaining the Square Mile's primacy as a financial nexus, then the site as a whole needs to offer engaging mixtures of sculpted scale and texture.

Before then, public space was strictly, and even officiously, regarded as something left over after planning - the acronym SLOP, signifying casual spatial vandalism, became widely used.Consider the substantial public square at the corner of Leadenhall Street and St Mary Axe. Give or take a few lunchtime sandwich-eaters sitting on the steps down to the plaza, it's a sterile, discouraging blank where the unique architectural syncopations of the City are lost, wiped clean by witless, cost-cutting redevelopment.The proposed new building on the site will be a physically dramatic counterweight to Lord Foster's Swiss Re building just across St Mary Axe. But it was only the redevelopment of Broadgate in the late 1980s that re-invented irregular, mixed-use architectural "porosity" in the City. And British Land is delivering a makeover in Leadenhall Street involving a slant-faced skyscraper concocted by Graham Stirk of the Richard Rogers Partnership.The challenge is about more than landmark buildings and dramatically pierced skylines. These sites must create a sense of place and set new benchmarks in multi-disciplinary urban design - and that's as much to do with groundlines as skylines.These may seem obvious requirements.

Minerva is redeveloping the junction of Houndsditch and St Botolph's, with a tower designed by Sir Nicholas Grimshaw. Pedestrianising Pall Mall East, between the square and the National Gallery, and connecting the two by a sweeping flight of steps, was nothing more than a tidy exercise in logic.A bit more than logic is required elsewhere, notably at two more critical sites in the City, which will pose a severe test to the amalgams of architects, spatial consultants and city planners involved. But the Royal Fine Art Commission savaged his offering as "an unpleasant Disneyland". There was architect-on-architect unseemliness, too, when Richard Rogers derided the same scheme as "false and nostalgic".The public remains dispossessed and uninvolved in these key changes to their urban fabrics. The remodelling of Trafalgar Square in London by Lord Foster may have been slavishly column-inched - but only at the expense of writing about more important tranches of the capital currently in transition. In 1987, Prince Charles, in effect the country's best-known architect, launched a stalking-horse into the fray in the form of a young, classically inclined architect called John Simpson. Of course it's a compromise in some respects, but if it weren't, nothing would have been built at all."The site has baffled waves of masterplanners and architects for 17 years.