Architecture: Edinburgh's invasion of the sugar castles: Britain's finest city is threatened by the arrival of a Post-Modern office development, complete with fake battlements. A lament by Jonathan Glancey

OF ALL British cities, Edinburgh is perhaps still the closest, at its centre, to architectural perfection. Although in the Sixties and Seventies it repeated the errors of stupid Sassenach towns, replacing Georgian and Victorian gems with brutally ugly commercial piles and ring-roads, still it ranks, visually, as one of the greatest in the world.

Yet the lessons of the past 20 years have not been learnt and, at the heart of the city, a tragedy is in the making. Princes Gate, a hotel and a heap of 'prestigious new offices to let', is erupting in a forest of concrete reinforcing bars at the foot of Calton Hill, one of the most characterful and unspoilt sites in, and sights of, the city. Recession has so far kept Princes Gate at bay; the development is rising from the ground slowly and is unlikely to be finished this year. But, as surely as Scotch chases ale in north-of-the-border bars, this ugly Post-Modern folly will take its place on the Edinburgh skyline alongside the exquisite Regency follies of Calton Hill.

The Princes Gate site shadows one side of Leith Street, the curving road that, in the shadow of Calton Hill, connects Waverley Station and the department stores of Princes Street - the city's main thoroughfare - with the Classical splendour of the 18th-century New Town. Along with the Scott Monument and Edinburgh Castle, Calton Hill is one of the key symbols of the Scottish capital. Rising several hundred feet above the city centre, this craggy mound is topped and tailed with monuments that encapsulate the city's history and aspirations.

On the top is one of the greatest of all architectural follies, the National Monument - 12 Doric columns, intended to be a replica of the Parthenon, which were built as a memorial to those who fell in the Napoleonic wars. Citizens refused to pay the pounds 42,000 needed to complete it, and, ever since, the bit that was erected has been known as 'Edinburgh's Disgrace'.

In the Seventies, this soubriquet was given by the people of Edinburgh to the crass, concrete St James Centre, a vast shopping complex that broods below Calton Hill and forms one side of Leith Street. Now the title should be passed on to the embryonic Princes Gate on the other side of the street.

On this significant and historic hill, a commercial folly of our age will rise that, unlike the gracious if 'disgraceful' National Monument of the 1820s, will be an eyesore, as if designed by wilful children bent on spoiling the existing order.

Leith Street will become a lost causeway; a street that, when the Princes Gate development is completed, could be in any city in the Western world.

Doubtless the developers of Princes Gate have done their homework and visitors to Edinburgh will flock to the new hotel like plovers to an estuary, while local businesses will snap up the office space to be fitted behind the sugary, Post-Modern skin of the complex.

But why litter the city that tourist brochures like to call 'the Athens of the North' with such facadist architecture, when the grain and character of the city is based on solid granite and a lack of fussy details? The new development is a long row of battlement- style offices - arcaded at street level - making full and fruity use of architectural details culled from various Edinburgh buildings. It tries far too hard to please.

Why the city's planners want to repeat the experience of the St James Centre - erecting a building that jars with the city, albeit in a different architectural style - is hard to imagine. To add insult to injury, the developers - Mount Charlotte Thistle Hotels - have chosen a firm of Sassenach architects (Covell Matthews Wheatley). With a site of such sensitivity, it might have been more appropriate to commission a local team of designers.

The story need not be so sad. There is a way of designing large, modern commercial and institutional buildings that will push the art and science of architecture forward and fit into the grain of historic Edinburgh. The answer lies under the developers', planners' and architects' noses on the other side of Calton Hill.

Here stands St Andrew's House, a castle-like, Modernist building of immense power, dignity and quality, designed by Thomas Tait, completed in 1939 and the home of the Scottish Office. Unlike the crude office and shopping developments of the Sixties and the imminent Princes Gate, St Andrew's House is at once modern and historic. In the outlines of its design it spells out Edinburgh as surely as William Playfair's neo-Greek High School (also at the foot of Calton Hill) or Edinburgh Castle itself.

Seen from the top of Calton Hill, from the street or between the Athenian vaults and Egyptian obelisks of Old Calton Cemetery, St Andrew's House is intimately connected with the city it adorns and the country it represents. Little more than 50 years old, it might have been here for 500. Thomas Tait's genius was to build massively and simply in granite - now dramatically blackened - so the building had a solid and reassuring presence.

Like nearly all the best buildings in Edinburgh, whether medieval, Georgian, Victorian or, more rarely, from this century, St Andrew's House is an intelligently proportioned mass of local stone that is punctuated by magnificent windows.

Approached closely, it is alive with contemporary details and architectural devices that tell the observer that this is Modern architecture, not some slavish copy or pathetic pastiche of what has gone before. Its window bay overlooking the cemetery might have been designed by one of the Bauhaus masters; the main entrance seems to draw its inspiration from Scotland's home-bred genius Charles Rennie Mackintosh as well from Bauhaus and Art Deco sources. The building appears to grow like some geological feature from its hillside site.

St Andrew's House is one of those rare buildings in Britain that has no difficulty reconciling the architectural obsessions of the present with those of the past. It is also a blueprint that could guide architects and developers as they wrestle with Edinburgh's future.

The favoured style of big Post- Modern developments - as the illustration on the developer's hoarding at Princes Gate shows - relies on decorative tricks to wheel their way past the planners. A hint of a battlement, a rusticated base and a variegated roofline are enough in the developer's and planner's eye to evoke the architectural vernacular of Edinburgh. And, yet, if you return to St Andrew's House, you will be hard- pressed to find an overtly Edinburgh or 'Scottish' detail.

Perhaps now is the time for the citizens of Edinburgh to rise up in protest. The city's beauty has already been sadly eroded. It is time to stop, look and demand something better.

(Photograph omitted)