The following correction was printed in the Guardian's Corrections and clarifications column, Friday 7 August 2009
In the design feature below, we accepted the claim that Hafod Eryri, the new visitor centre on Snowdon/Yr Wyddfa, is the UK's highest inhabited building at about 1,069m, or 3,507ft. In fact, that is topped by the Ptarmigan station restaurant on Cairn Gorm at about 1,097m, or 3,600ft.
On a clear day, standing on the top of Mount Snowdon, you can see right across the Irish Sea. The views from the Welsh peak – which, at 3,560ft (1,085 metres), is the highest British mountain south of the Scottish highlands – are breathtaking, and the weather is savage. In winter, winds can reach 150mph, while the temperature plummets to -20C. In summer, it can reach more than 30C. Yet here you will find Britain's unlikeliest (and highest) building.
For 70 years, the cafe that stood here was an increasingly shabby, weather-beaten concrete hut; in its latter days, it was described by Prince Charles as the "highest slum in Wales". This bunker was the work of Clough Williams-Ellis, better known as the architect of Portmeirion, the dreamlike, Italianate holiday village overlooking the tidal waters of Traeth Bach and Tremadog Bay some 10 miles south. (It was the location for the 1960s TV series The Prisoner.)
I have no idea how Clough fluffed it so badly when it came to Snowdon – why erect an architectural eyesore amid such natural beauty? – but the story has a happy ending. The "slum" was demolished in 2006 and in its place the Croydon-based architect Ray Hole has come up with a minor masterpiece: Hafod Eryri, the new Snowdon Mountain railway terminus and visitor centre. There are two ways to reach the top of the mountain: on foot or by train, via Britain's only rack-and-pinion mountain railway. I walked, for three hours, in soft, insistent rain with tempestuous clouds scudding ever lower. The stone- and slate-faced building at the top came as a complete surprise. It is both modern and ancient; it is elemental, and now a piece of the Snowdon landscape, a part of the mountain it serves. It hunkers down into the rocks. The walls and roof are curved to fit Snowdon's contours and to counter the tumultuous winds and rain.
And yet some believe Hafod Eryri (which translates roughly from the Welsh as "summer residence in Snowdonia") should never have been built at all. When the decision was made to rid Snowdon of its concrete bunker, there was discussion as to whether to replace it. Conservationists argued that it would be better to return the peak to nature.
But tourists have been making their way up to Snowdon's summit since at least the 1780s, and the Snowdon Mountain Railway – based on German and Swiss engineering practice, and running four Swiss-built steam locomotives (as well as some 1980s diesel engines) to the peak from Llanberis station at the mountain's foot – has been here since 1896. In fact, when the railway opened, there were two rival hotels on the summit. One blew down, while the other had vanished by the mid-1930s. But Snowdon remained a tourist magnet and it needed, somehow, to cater for hungry, thirsty and weather-beaten souls.
The astonishing thing is that the £8.4m building has gone up so quickly. Construction began in 2007, with a team of at least 80 people working in what have often been appalling conditions. The workforce made their way up to the site, along with all the materials, by train. In winter, trains had to be dug out of the snow and, in the worst instances, builders had to trek back down the mountainside after a hard day's work. The building's frame had to be erected as quickly as possible, and the walls and roof clad in granite and slate in order to shelter the construction team from the savage elements. Nor is the building itself completely immune to the weather: Hafod Eryri will close when the summer season ends. The windows will be closed and shuttered, and all the water drained away. As the snows fall, it will become little more than a picturesque rock pile. This, truly, is architecture in extremis.
Hafod Eryri stands on the same foundations as the 1930s cafe it replaced. Its steel structure is clad with aluminium and then faced in Portuguese stone. Why not Welsh stone? There is some, from nearby Blaenau Ffestiniog, but the curious workings of the global economy mean that it has been cheaper to ship stone from across the Bay of Biscay than to cut and hew it from Snowdonia itself. The stones are finely laid, and animated on the exterior by five carved stanzas composed especially for the building by the poet Gwyn Thomas. "The summit of Snowdon, here you are nearer to heaven", reads one. "All around us are the grandeur and the anguish of an old, old nation", says another.
Inside, there is a handsome room set under a sail-like roof. Huge, aluminium-framed, west-facing windows offer thrilling views that are almost impossible to take in, particularly for visitors who are getting off one train and taking another back down the improbable gradient (an average of 1 in 7.6) to Llanberis. The windows are inclined to offset the solar glare – although I have to admit that I only managed to contemplate the entire operatic view for a few minutes, before clouds enveloped the building and the rain set in with a vengeance.
Even so, this is a pleasant enough place in which to hole up once the crowds have thinned. The walls are lined in timber, the floors in slate, and the overall feeling of quality and solidity is reassuring when you remember that you are on top of a mountain. Once the railway passengers and walkers have retired for the day, most of the staff settle into the building for the night. Hafod Eryri has to be both comfortable and comforting: summer workers spend virtually the entire season at the summit (it would be all but impossible to commute).
Ray Hole is the creator of a number of excellent visitor centres, including the Bentley Pavilion at Volkswagen's Autostadt at Wolfsburg in Germany, sunk underground through a seductive green granite shell; the Rainforest House in Hanover, which resembles a miniature Eden Project; and the designs for a museum in Port Elizabeth, South Africa, devoted to the anti-apartheid activist, Steve Biko. Both architect and structural engineer, Hole is adept at shaping buildings in unusual or difficult settings. Hafod Eryri is the best of the bunch; it seems something of a shame that the majority of visitors who come this way are in and out of the place within minutes.
When I take the train back down to Llanberis, nearly five miles and an hour's ride below, the new station and visitor centre vanish out of sight in little more than an instant. The last train of the day is left to hiss gently down the mountainside, offering us a final view of Snowdon and the valley below, before the veteran locomotive and its single coach are swallowed up by trees, and the peak vanishes.
The fact that Hafod Eryri fits so effortlessly into that peak is a real credit to those who designed and built it. I had wondered, at the top of the mountain, what the people pouring off the trains thought of the building. I asked about 20: three went as far as saying the building was OK; others thought it was much the same as it had been before. Clearly, and despite a truly heroic design and building process, there's no pleasing some folk – even up here, in the realm of the Celtic gods, engineering sorcery and magical architecture.