Edinburgh's new stronghold

The unfinished, raw concrete building swarmed over by builders with hard hats and steel-capped boots doesn't look its best. The National Museum of Scotland in its final phase of construction reveals its skeletal bones without so much as a plaster skim, and the scaffolding blocks both the light and the lines. But, like a supermodel pre-makeup, long before the stylists have gone in, and even with major surgery underway, this building cannot hide the fact that it is stunningly good looking, as refined as Miss Jean Brodie long before its prime.

"For us, it's best before it's tamed, a tiger not a pussy cat," says the architect, Gordon Benson of Benson and Forsyth. In truth this building, a shrine to over 10,000 everyday household artefacts through Scottish history, will never be domesticated. It's always going to be an exotic creature. Wannabe Pevsners have already hailed it as one of the major post war buildings in Britain, simply from the dreamy watercolours. Lord Rothschild, normally a man of few words, eulogised it as "one of the great, great projects of the last decade".

Scotland's newest and greatest modern building fits seamlessly into the fabric of Edinburgh's old town - it has all the stolidity and grace of a medieval castle with Corbusian configurations. The light wells within, for example, are pure Corbusier at Ronchamps. "The trick was to link the castle keep with a strong curtain wall of stone," Benson says. "And to use the Cartesian grid, but to hide it in some parts which have the irregularity of the medieval parts of Old Town in Edinburgh. The building is not a geometric vanity."

Edinburgh is an astonishing city built around an extinct volcano, Arthur's Seat, and sporting a medieval castle. The architects respect this by taking elements of castle architecture such as the narrow, arrow-slit windows. Despite the architects' admiration for this strait-laced city, their building has a delirious freedom. The lower outer building wraps around a larger, rectangular box which stands at the core of the building. Narrow triangular shafts in one vertiginous ascent open out into this core, like a castle keep. A glorious tower at the north-west corner echoes Edinburgh's half moon battery, visible from the roof.

There are roof parapets and narrow slit windows that beam light into the interior. At the core is a light well, where the large steam engines and exhibits from trade and commerce and industry are displayed. A narrow skylight runs around the perimeter of the core gallery and light falls down a slot the full height of the building, so that at all points of entry the natural light is beamed in under controlled conditions.

Benson is clear about the importance of respecting 18th and 19th century urban planning. It's what made him soul mates with his partner, Alan Forsyth, when they met at the Architectural Association. Richard Rogers and his partners, John Young and Mike Davies, were students there at the time, "like moon men with shaved heads who loved those lunar landings. We admired Victorian 19th century urbanity, loved Corbusier's buildings, but loathed his town planning proposals".

Materials both inside and outside the museum have the same simplicity - great swoops of cast concrete so beautifully rendered they feel like silk to the touch, with floors and outside cladding in buff and brown Clashach stone, still quarried at Elgin. The reason it's unlike any other museum is that every element has been planned to tailor together, like haute couture, not built as a package with different contractors working apart. The single-minded identity that Benson Forsyth give their buildings is rigorously applied. Every builder on site had to be right-handed so the movement and sweep of their handwork runs in the same direction.

With fastidiousness like this you can imagine the care with which they approached Scotland's history, revealed

through the collection of objects like Bonny Prince Charlie's cape and a piece of bone and alabaster from his tomb. Or Mary Queen of Scot's earrings and letters, Rob Roy's purse and spoon carved from horn. This collection carries a lot of national pride.

The current thinking in the Museum of Scotland is that displays should be seen to be spare, with simplicity and directness. This is a good decision because it allows the viewer to experience the architecture. And, like everything done by Benson and Forsyth, their architecture needs to be experienced. Take the white cube of an oratory they designed in a hospice run by the Augustinian nuns in Cumbria: truly luminous, this white building made of etched glass is a place of arresting beauty. "All feeling, no function," Gordon says modestly. On one side the building frames the silvery sands of Morecambe which reflect in their watery surface the sky. On the other the beech trees raise their fingers to Heaven. Whatever your beliefs, this is a place that takes the onlooker outside themselves, evoking that much overworked word, spirituality.

In the basement, there are exhibits from the first arrival of humans around 7000 BC until the Norse settlements around 1100 AD. The ground floor is the Kingdom of the Scots 1100-1707 and Industry and Empire from 1707-1914. All the social, economic, political and religious developments experienced in Scotland are depicted. "One of the reasons we experience museum fatigue is by curators adopting the posture that we are the experts. It's a one-way dialogue through a megaphone." David Clarke, curator of Early Man, is determined his section isn't seen as a barbarian stronghold with skins on the walls. So he commissioned Andy Goldsworthy to make four semi-circular slate walls in the basement as the backdrop for man as hunter gatherer.

"Visitors bring their imagination," says Clarke. "We give them the context and the props." His Viking hoard of120 pieces has silver thistle brooches and neck bands and 10th century coins from Samarkand, Tashkent and Baghdad. He also has enough equipment to requip the Roman army.

Pre-Christian is the dark ages - darkness, but with shafts of light penetrating it. As you progress upwards through the building due east, the windows get bigger. The medieval church is like a crypt, with precious objects shown in a low light against stone and natural concrete. At the Reform stage the sky begins to be seen. and you enter the west-facing room where the Age of Enlightenment is bathed in natural light.

The curator for the 1100-1707 period, George Dalgleish, explains that showing things in an honest way to Braveheart filmgoers means they can experience a great chunk of history in a real life context - every bit as exacting as Disney, but with more integrity

When it comes to the Industrial Revolution - when the Scots had more interesting things to do than fight with the English - the space opens out, and is highly vertical and lit overhead. There are gigantic steam engines, and to celebrate the moment when water becomes steam, the structure of the building is revealed: columns emerge. In the middle is a real-life, cruck frame house with its peat roof insulated with bundles of heather.

Among the grinding cement mixers, and in a slipstream of sparks from welders, these bunches of heather fumigated to get rid of any "wee slikit timorous beesties" are a surprising reminder of the effectiveness of building materials of the pastand also of the egalitarianism of the museum collection. A fragment of the panelled, painted walls from an Edinburgh house from the Age of Enlightenment, when economist Adam Smith and philosopher David Hulme were changing the way we thought, is built into the wall.

At the top of the building in the 20th century gallery the interior is flooded with light. It contains rather banal items like electric blankets and milk bottles, selected by the public who were invited to submit ideas for what should be in this temporary display. Museum director Mark Jones also asked people to select things they couldn't live without. Tony Blair chose an electric guitar. Sean Connery chose a piece from the Declaration of Arbroath from 1320, the bit where Scotland will never yield to English domination.

Jones singles out the camera - "compromising our way of seeing things" - and the motor car, neither of which he sees as progress. Nor does he see progress itself as necessarily desirable. In blaming the car for the way in which roads carved up cities, and communities, Jones shares Benson's vision. In their own ways they both believe that technology separates us from natural and immediate experience. That must be why between them they have created the finest museum this century.

What makes the celebration of Scotland's culture and heritage so profoundly poignant is that Benson & Forsyth have not been shortlisted to design the Scottish Parliament. They were in there for the first jump-off but didn't make the final round. The five who did are Richard Meier, straight from the Getty Museum in San Francisco; Enric Miralles y Moya, who reminds us (and the Secretary of State for Scotland, Donald Dewar) with his Catalan nationality that were there to be a national assembly in Catalonia it would be unthinkable not to have invited a Catalan to participate; Rafael Vinoly, who built a billion-pound Tokyo forum; Denton Corker Marshall from Australia, which is a bit out of the way, and Michael Wilford, the winner of this year's Stirling prize.

The Scottish Office have missed a very important opportunity to get a building that will not lend itself to superficial imagery. The Benson & Forsyth museum illustrates the value of separateness and what flows from that. It's not nationalism so much as national pride.