The splendid fortifications of Carcassonne cannot hold back a tourist invasion. By John Watkins
In 1844, a French architect, Viollet-le-Duc, and an archaeologist, JP Cros-Mayrevieille, began the reconstruction of a crumbling fortress at Carcassonne, in south-west France. Derelict shops and houses were refurbished and 50 rounded towers with pointed roofs graced the city's walls once more. In time, coach and car parks were built, a tourist office was installed in the Porte Narbonnaise and a train dropped yet more visitors outside the imposing drawbridge.

A Cathar stronghold during the 12th century, Carcassonne fell to the Albigensian Crusade in 1209, and then for 400 years it guarded the Franco- Spanish border - until the latter moved south. Of course, by the time it had been restored, the splendid fortifications no longer had any military function. In a perverse reversal of history, the walls now attracted rather than repelled visitors, who swarmed to Carcassonne in their tens of thousands to experience for themselves the splendours of a medieval fortress. During the summer months, they thrilled to the medieval equine skills on display in the "live show of Chevalery", and in the velvet evenings, they sat entranced as the night sky above them was criss-crossed with laser beams as part of the son et lumiere.

For those of a more historical turn of mind, the Torture Museum detailed the methods used by the Inquisition to root out the Cathar heresy, and in the museum of Le Moyen-Age dans la Cite, a series of wax tableaux depicted life in the Middle Ages, with explanatory texts on the wall (and piped music).

Noting the visitors' insatiable appetite for souvenirs, the shopkeepers obligingly provided them with a bewildering array of relics. Nowhere more so than on the rue Cros-Mayrevieille, which leads up from the Porte Narbonnaise, where, like their medieval ancestors, fretful visitors clogged the narrow cobbled street. But unlike the Inquisitors who burned Cathars, these late 20th-century pilgrims preferred to burn money. Some enterprising traders appended "Le Vieux" or "Cathar" to the name of their cafe or boutique, fearing that if they did not make sufficient obeisance to the medieval myth, its economic potency might be damaged.

But don't give up - if you survive the crush and reach the top of the rue Cros-Mayrevieille, the congestion eases. The road opens into a small square outside the castle, and as you sip an outrageously expensive drink in the shadow of its walls, you begin to notice that within this madness there are subtleties of atmosphere. The south-eastern corner of the city is devoted to eating and is necessarily less frenetic. The large, shady square is filled with restaurant tables; around the perimeter, snack bars sell waffles, crepes and drinks. Piped medieval music aids digestion.

Around the Cathedral of Saint-Nazaire, there is an altogether more up- market ambience. Here the restaurants have their own gardens and terraces. A night in the ivy-covered Hotel de la Cite will cost you between 900 and 1,750 francs. But at least you know that if you cannot sleep, the bookshop next door has a wide range of serious reading on the Cathars.

And if the bustle should become unbearable, there is space and quiet aplenty in the Listes - the broad, dusty avenue between the two sets of walls. In times of war, this is where intruders who had breached the outer wall were caught and skewered. In our own, more predictable times, rich visitors can drive up and down in a hired horse and trap.

The Inquisitors who burned and tortured the Cathars into submission might well recognise the intensity of today's commercial activity. And although the ascetic Cathars would not have looked kindly on the wealth and luxury on sale, they too might have smiled wryly at the doctrinal purity underpinning the single-minded imperative to sell.

Paradoxically, the very precision of its restoration undermines Carcassonne's credibility. Its completeness denies the visitor any imaginative space - it's all done for you. Those raised on Disney-style images of fantasy castles might find it hard to believe that this was once a real city. Seen from the south in the evening, there is a fantastical opulence about Carcassonne; a mirage of crenellated walls and towers suspended above the vineyards too outrageous even to be dreamt up by a jaded Hollywood director.

From this perspective of perfection, the panorama occupied by one of the finest medieval fortresses in Western Europe, I began to fret. In thrall to the material world, we have managed to reduce Carcassonne to a single dimension, little more than a reason to buy things. Perhaps what Carcassonne tells us about the medieval world is that the gulf between us and them, between now and then, is all but unbridgeable.