Novelist Kate Mosse braves chill Pyrenean winds to tour the land of the Cathars, whose grottoes sheltered resistance fighters and which even today enjoys a wild, remote charm

The Ariège lies in the Pyrenees, less than an hour from Toulouse, east of the Lourdes pilgrim trail, bordered by the Aude, the Pyrénées-Orientales, the Haute-Garonne and the tiny independent state of Andorra. Named after a torrential river, the Ariège is a wild place, a landscape of legends, of gorges and rocks that loom over its major towns; Mirepoix, Foix, Tarascon, Saint-Girons and Ax-les-Thermes.

It is France's least populated département, the poor relation of the tourist towns of Carcassonne, Montpellier and Albi. Neither entirely French, nor quite Spanish, the Ariège is uniquely and quietly itself. It is beautiful year-round - mild springs, hot summers - but in winter, when the low sun glints off the foothills, it's the perfect place for relaxation, adventure or shopping. Like all mountain regions, it's also fiercely proud of its traditions.

We flew into Toulouse on a bitter Tuesday morning. The sky was grey, and an Arctic wind was chasing around the airport terminal. It was coffee time, but the temperature was only just nudging zero, so I pulled an absurd (and itchy) woolly hat over my ears and added an extra pair of socks before setting out to pick up the hire car.

The ring road around the airport is fiendish - tall, imposing walls of concrete and signs that only appear seconds before the turnings. But within half an hour we had negotiated the péage and were on the A61 heading west, following the line of the Canal du Midi, on the Autoroute des Deux Mers. The land to the left was flat, filled with parcels of land. Behind the railway, stark rows of silver birches seemed to keep guard in the distance. To the right of the motorway, there were already signs of the mountains - gentle hills, more like Sussex than the Midi, with clusters of houses, churches silhouetted against a winter sky. Everywhere, heritage road signs declared that we were in Cathar Country.

Rather than head straight for Foix, the capital of the département, we decided to go via one of our favourite towns, the pretty, medieval bastide or walled town of Mirepoix. Besides, it was fast approaching lunchtime so the thought of a glass of local rosé seemed a quicker way to get into the distinctive character of the region than sightseeing.

Wrestling with the hire car's heating and radio stations playing French rap and Europap, we turned on to the A66 and drove due south towards Pamiers, birthplace of the composer Gabriel Fauré, then swung west to join the D119 to Mirepoix itself. It was a desolate piece of road, trapped between the colourful hills and valleys around it. The airstrip at les Pujols looked all-but deserted, the houses shuttered and cowering in the wind.

Twenty minutes later, the red-tiled roofs and imposing stone graveyard on the outskirts of the town, signalled we were back to civilisation. Mirepoix is constructed around a large central square, and used to stand on the right bank of the Hers, at the foot of the castle of Terride. In 1289 the town was flooded and only the castle survived. When the town was rebuilt 10 years later, it was on the left bank. In the summer, it's jam-packed with tourists, but at this time of year, you'll hear more French than English spoken. We parked on the outskirts, then walked down cobbled pathways towards the medieval centre.

Mirepoix is famous for its beautiful, colonnaded square, however, the Place du Maréchal Leclerc is dominated by the Cathedral of St-Maurice. Like the poet Philip Larkin, who was unable to pass a church, I found myself pushing open the heavy wooden door and stepping in, despite my ambivalence about places of worship - especially here in the Languedoc where the scars of the 13th-century religious wars and conquest of the South by the North run deep. But unlike many of these southern cathedrals - St-Sernin in Toulouse, where the Inquisition kept its tortured, broken prisoners, or the Basilica St-Nazaire et St-Celse in Carcassonne, where a sense of disquiet hangs in the air as heavy as incense - St-Maurice is welcoming. And on this bitter day, having lost the feeling in the tips of my fingers, it was also, more importantly, warm.

Like many cathedrals in the Midi, it is a mix of architectural period and fashion. The foundations date from the 12th century, but it was enlarged little by little from 1327 to the 16th century.

Inside, as I wandered through the nave - which is the widest in France (at 22 metres), and also the second widest in Europe - fragments of melody curled round the stone pillars, the same bars practised over and again by the organist.

After a leg of lamb for my husband and a toasted goat's cheese dressed with local honey for me (being a vegetarian is tricky in this part of France), we were back on the road, driving towards Lavelanet. As we reached the outskirts, I caught my first glimpse of the snow-capped pog (rock formation) of Montségur, with the ruined citadel sharp against the now clear, blue sky. It was there in March 1244 - after several months of siege and the slow death of hope - that the Cathar church was finally defeated and the independence of the South was lost. Even from this distance, there is a sense of tragedy that hangs like a shroud around the citadel, magnificent and defiant still. This is not the time of year to venture to the summit. The winds are too bitter, the snow too deep. Even down here in the valley, there was a dusting of white framing the roadside, like icing sugar. On higher ground, the fields were covered.

In January or February, you can take the Montferrier road, which leads to the most terrifying, switchback 30-minute drive up a vertiginous mountain road to the most family-friendly of the 10 Ariège ski resorts, Le Mont d'Olmes. But we were in search of a less ordinary adventure.

My destination was the magical caverns of the Grotte de Lombrives, south of Tarascon-sur-Ariège. We took the N20 towards Andorra, an elegant, sweeping road that follows the sparkling valley of the Ariège itself. This is a stunning landscape of jagged crests with stark, singular stone profiles, of labyrinthine caves and subterranean rivers and streams. The natural landmarks have names: le Trou du Curé (the Priest's Hole) or le Pont du Diable (the Devil's Bridge), echoes of the old, forgotten stories of the Ariège and of the demons and devils that are believed to haunt these mountain passes. The further south we drove, the more the mountains crowded in around us. At Tarascon, they were so close that the clouds hovered, almost within reach, between the crags and promontories.

The Ariège has the largest concentration of grottoes in France. In the 13th century, Cathars hid from the Inquisition, sometimes living in the caverns for years. During the 17th-century wars between Spain and France, the caves were used for prisoners and as hiding places. In the 20th century, Jews, resistance fighters and Allied airmen sheltered in the underground passages until they could escape over the Pyrenees to Spain.

Within 45 minutes, we had arrived at the neglected railway sidings that serve as the car park for the Grottes de Lombrives. With uncomfortable memories of MR James ghost stories rattling around in my mind - his very first tale, "Canon Alberic's Scrapbook", was inspired by a visit to a tumbledown church at St-Bertrand de Comminges in the Pyrenees - we set out for the entrance to the caves. We picked our way along an overgrown path, bounded by derelict buildings. An old wooden sign advertising the opening hours was the only indication that we were in the right place. The ticket office was just a small wooden hut. But despite the air of disuse and lack of interest, it was open.

Pulling my hat down even lower and my collar up even higher, I joined a rickety old train pulled by an ancient tractor. I hate fairgrounds, I hate speed and I hate heights, so I kept my eyes shut. In the chill afternoon, the entrance to the caves was illuminated in a shaft of light. I felt as if I'd walked into a painting by Dürer or one of the Pre-Raphaelites. The silence of the mountain pressed in. This was not tourism; more a step back in time.

Year round, the temperature within the Lombrives caves is 13C; never more, never less. The grating of an old metal door drawn back, the steady drip of water, the uneasy whispered conversations of the other visitors, and we walked down in single file. The door clattered shut behind us.

There's a spirit of place in these ancient caves that always makes me catch my breath. I followed the guide, the light from his torch dancing on the roof and walls; I was half listening and half soaking up the atmosphere. In the tradition of trying to tame such ancient places, all the caves have been given names. The most beguiling is the Tombe de Pyrène, the legendary Princess and lover of the demi-god Hercules, after whom the Pyrenees are said to be named. The tomb itself is a beautiful white stalagmite shaped like a burial mound. True to the legend, the granite roof never stops weeping its opalescent calcaire.

When we emerged from Lombrives, dusk was falling. It was that time, entre chien et loup, when there's still a hint of the day left in the sky. Street lamps and car headlights on the mountain roads sparkled far below. The ancient forest of fir and birch was silhouetted against the skyline, erect and alert like a line of soldiers. I closed my eyes again on the way down and thought of Foix, our next stopping place, reminding myself that no one has yet died on this mountain.

Foix, the departmental capital, sits in the centre of the Ariège like a spider in its web. Sheltered in a natural valley where the rivers Ariège and Arget meet, this has long been a site of great strategic importance - from prehistoric times, through the Visigoth and Merovingian eras, to the wars of occupation of the 13th century. There is plenty of interesting modern architecture - especially an astounding 1930s art deco post office on the main street - and a well-preserved medieval centre.

Foix is a city dominated by its castle. Where the ruined citadel of Montségur speaks of gods and giants and kingdoms in the clouds, the castle at * * Foix proclaims the might of men. In the Albigensian Crusade of the 13th century, it was the only fortress never taken by the northern French invaders. The foundations of the current castle dates from 1000. The chateau sits, dominant, splendid, powerful, on top of a rock that looks as if it might have fallen from the sky. The climb up to the chateau itself is not for the faint-hearted. A surprisingly discreet wrought-iron gate at the bottom leads to steps, then you find yourself on an open, steep, cobblestone ramp that zigzags up the side of the mountain on which the chateau sits.

But it's worth it. The view from the top is breathtaking. The rivers, the lights, the church of St-Volusien down below, and the castle itself. There are two distinctive square towers which date from the 12th and 13th centuries, as well as the 15th-century round tower, which boasts an Errol Flynn-style winding stone staircase and allows you to climb to the very top, weather permitting.

Since 1930, the chateau has been the home of the collection of the Musée Departémental d'Ariège. There are artefacts dating from prehistoric times, Gallo-Roman remains but if, like me, you're a sucker for swords and all things medieval, best of all is the fabulous collection of armour and weapons. For a while, I sat in silence on a metal bench, writing in my head scenes for my next novel - this is the perfect place for a murder - until thoughts of supper and the realisation that my toes were frozen stiff in my boots called me back.

I headed for Les Halles de St Volusien for the regular Tuesday afternoon market. There was jewellery and sheepskin slippers, bottles of the local aperitif, Hypocras - a bittersweet spicy drink from a medieval recipe made from wine, cinnamon, ginger, cloves, cardamom and rose petals - as well as jars of local acacia or chestnut honey, honeycombs and handmade sweets.

I ambled between the stalls, looking at the free-range geese, turkeys, capons and guinea fowl. Local gourmets can also enjoy sanglier - wild boar - a dark, tender meat which I'm told tastes like gamey beef. Fresh vegetables, of course, can be found here, as well as foie gras, for which the south-west is famed.

We'd decided to stay for the night and catch a flight back from Toulouse in the morning, so we found a pleasant hotel in the centre, then booked a table at our favourite restaurant, Le Phoebus. It has a wonderful glassed-in terrace overlooking the River Ariège and a panoramic view of the castle. Specialties include foie gras, profiteroles, asparagus mousse, spice-roasted duck breast with shallot confit and curried turnip purée.

Ever cautious, I chose a perfect, buttery omelette, while my husband had cassoulet, a traditional Languedoc pork, duck and white-bean stew, washed down with a bottle of the local rosé, which is strong, more like sherry than the sweeter rosé of the Minervois. As an aperitif, a glass of Guignolet, a bittersweet local cherry liqueur, before we quit the fire for the short, cold stroll back to our little B&B.

As I climbed into bed, the bells of the cathedral struck 11. Pleasantly exhausted by the mountain air, I was asleep before they struck midnight.

TRAVELLER'S GUIDE

GETTING THERE

The main gateway by air is Toulouse, served by British Airways (0870 850 9 850; www.ba.com), Flybe (0871 700 0123; www.flybe.com), easyJet (0905 821 0905; www.easyJet.com) and BMI (0870 60 70 555; www.flybmi.co.uk). Ryanair (0906 270 5656; www.ryanair.com) flies from Stansted, Nottingham and Liverpool to Carcassonne, an alternative gateway.

GETTING AROUND

From Toulouse there are direct trains to Pamiers, Foix and beyond. Rail services are supplemented by buses, but these run infrequently - especially at weekends.

STAYING THERE

La Maison des Consuls, 6 Place du Maréchal Leclerc, Mirepoix (00 33 5 61 68 81 81; www.maisondes consuls.com). Doubles start at €75 (£54), room only.

Hôtel-restaurant Relais Royal, 8 Rue Maréchal Clauzel, Mirepoix (00 33 5 61 60 19 19; www.relaisroyal.com). Doubles start at €150 (£107), room only.

Hotel Pyrène, Rue Serge Denis, Foix (00 33 5 61 65 48 66; www.hotelpyrene.com). Doubles start at €51 (£36), room only. The hotel is closed until 1 February.

VISITING THERE

Grotte de Lombrives, Ussat-les-Bains (00 33 1 39 55 58 53; www.cathares.org/lombrives.html).

Le Château de Foix, Foix (00 33 5 34 09 83 83; www.ariege.com/chateau defoix). Admission €4.20 (£3).

EATING AND DRINKING

Le Phoebus, 3 cours Irénée Cros, Foix (00 33 5 61 65 10 42).

Le Cantegril, 25 Place Maréchal Leclerc, Mirepoix (00 33 5 61 68 11 26).

Le Bellevue, 7 Place Jean Jaures, Tarascon (00 33 5 61 05 52 06).

FURTHER INFORMATION

Mirepoix Tourism (00 33 5 61 68 83 76; www.ot-mirepoix.fr).

Lavelanet Tourism (00 33 5 61 01 22 20; www.paysdolmes.org).

Foix Tourism (00 33 5 61 65 12 12; www.ot-foix.fr).

Montségur Tourism (00 33 5 61 03 03 03; www.montsegur.org).

Tarascon Tourism (00 33 5 61 05 94 94; www.tarascon.org).

Ariège Tourism (00 33 5 61 02 30 70; www.ariegepyrenees.com, in French; www.ariege.com, in English).

French Government Tourist Office (09068 244 123, calls 60p/min; www.franceguide.com).

'Labyrinth' is published in paperback on 11 January, (Orion, £7.99) and has been chosen for Richard and Judy's Book Club on Channel 4

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