After 20 years of living and travelling in Spain, I like to think I have a handle on the country and its people. Every so often, however, they can still spring a surprise.

Like when Guillermo Maana, a 70-year-old scholar, first told me about the 56-kilometre Camn Real de la Mesa. The Camn Real, said Maana, was an ancient trail through the mountains of northern Spain, winding spectacularly among some of the grandest yet loneliest and least-known sceneries in Europe. I had never heard of it but if I was up for it, he said, he would show me the secrets of this magical route.

Roman remains

For centuries, the Camn was one of the few points of contact between the provinces of Len and Asturias. It is essentially Roman in construction but the route has been used for trade for 5,000 years, traversing a mountain range with peaks of 2,000 metres, reaching into some of Spain's most wildly beautiful and otherwise inaccessible landscapes.

I was gripped by Maana's vision of this long and winding road, its historical importance and its near-obliteration at the hands of modern life. So we arranged a two-day trek on the section of the way that is accessible only to walkers, leaving out the northerly part which has been covered with asphalt, its beauty spoilt.

Empty roads

Our route would take us from Torrestio, at the northern edge of the province of Len, to the village of Dolia in the county of Belmonte, Asturias — a distance of some 30 kilometres. At both ends of the route there would be simple places to stay but the Camn passes through no other village, so the plan was to take food and a sleeping bag. In summer you can sleep under the stars or take a tent but, since it was autumn, we would bed down in one of the thatched shepherds' huts, called teitos.

We met up on Sunday night in the mountain town of San Emiliano and dined on fried eggs in the Hostal de Montaa, a simple mountain hostel. Before dawn next day, we drove to the hamlet of Torrestio, under a dark sky as cold and clear as spring water. At 7am, there was a blanket of mist over the valley. We set off in the half-dark, heading up the Valle de las Partidas: the Valley of Departures. Up ahead, the first rays of the Sun were beautifying the squat grey peak of El Mun.

At the top of the valley was a fence marking the border between the two regions, Castilla y Len and Asturias. A concrete pillar gave the height above sea level: 1,782 metres. To the north lay a wide stretch of pasture between mountains: the Mesa, or tableland, from which the Camn takes its name. Brown cows with wide horns stood and stared as we passed and the quiet was blurred only by waterfalls and cowbells.

Further down the Mesa lay a scattering of stone huts. These hamlets, called braas, are the only human settlements in these mountains. We stopped beside a waterfall for lunch — Asturian cheese, black chocolate and bread with olive oil and fresh spring water.

A shepherd came by looking for a lost foal. A pair of binoculars hung around his neck and by his side was a dog as big as a small pony. He feared the worst: Last spring, four or five of his horses had been taken by wolves.

As we walked, Maana pointed out historical, natural or architectural sights along the way, ranging from a wide meadow called Xuego La Bola to a long trench that had been an 8th-century defensive wall during the reign of Alfonso ll.

Historically rich

Bronze-age burial mounds can be seen along the route but it was the Romans, or rather, their slaves, who built a proper four-metre-wide path. By the third century AD, it was the main access route between Len and Asturias, used primarily by Roman civil servants and gold dealers heading south from the mines of Belmonte.

Then in the early 19th century, a trunk road was built linking Len and Oviedo via the Pajares pass and the Camn fell into disuse. But it remained a secret door into the stunning wilderness of the Somiedo reserve.

Roman road-building skills made the Camn a broad path with a modest gradient. The walk is never gruelling but the views are spectacular. On the far horizon lay a line of palest blue: the Cantabrian Sea. After eight hours, we stumbled into the shadow of a strange crag, La Pea Negra (the black rock), dark and sinister.

Braa La Corra, a collection of seven roughly thatched stone teitos, were deserted but in reasonable condition. Shepherds live there in summer but walkers are free to use any left open, though they can't be reserved.

From the terrace of our rustic lodging, 1,200 metres up, we could see the deep Valle de Saliencia below us and glacial lakes to the south, among a bristle of ash-grey peaks. The thick forests opposite are one of last remaining habitats of the Cantabrian bear, of which some 130 remain.

At 7pm, night fell like a stone and so did the temperature. We cocooned ourselves in our sleeping bags and Maana told me stories about the Camn Real, its history and legends.

The next day we discovered the remains of a venta, a small stone shop in a wide green pasture called Piedra Jueves (Jupiter's altar) that once sold vinegar for the feet to shepherds who had travelled for days to bring their sheep to the spot.

Once upon a time ...

We stood at the crest of the hill, surveying an Impressionist wash of grey-green broom, yellow birch and a scarlet stipple of rowan berries. I looked in vain for a building, a road or a human figure, but there were none.

The floor of the valley was speckled with bleached heaps of stone, which, centuries before, had been dwellings. Maana, who had known the Camn as a populated place 35 years ago, told me about a great livestock fair that had been held annually up here, 1,000 metres above sea level.

"Do you see now what jewels we have and what a state they're in?" Maana asked bitterly.

At Cueiro, the Camn diverges east towards Oviedo (the Camn Francs) or north to Llanera and Gijn. We struck north, passing a large former venta, now a barn, ripe for conversion into a simple B&B for walkers unwilling to sleep on a floor covered with hay.

The village of Dolia was pretty and bucolic, snoozing amid hazel woods, but the asphalt underfoot and the power lines overhead came as a shock after our three days in the wilderness.

We had covered three quarters of the 40 kilometres that can still be walked. The last quarter, where it pushes into the 21st-century world of petrol stations and builders' merchants, has lost its mystery.