National treasure house.

Crafted with love and sensitivity for its location and for its poignant contents, the building that tells Scotland's story is a major and popular contribution to the cityscape of Edinburgh and development of museums as a type.

Until now, no decent public buildings have been made in Edinburgh since Robert Lorimer built the Scottish National War Memorial on the Castle Rock in the 1920s. With its amazing site dominated by the two extinct volcanoes: the castle crag, and Arthur's Seat (a mountain in the middle of the modern town), Edinburgh is one of the most daunting places to design in that can be imagined. Particularly so, as at the turn of the eighteenth century in an extraordinary burst of prosperity and cultural self-confidence, the Scots made their capital the finest NeoClassical city in the world - the Enlightenment was revealed in three dimensions. Craig, Adam, Playfair and their contemporaries were almost impossible to follow, and for 150 years their gigantic shadows overwhelmed architectural creativity. Relative loss of prosperity compared with Glasgow generated a pawky, Presbyterian provincial culture in Edinburgh, run by canny clerics, lawyers and bankers, vastly different from the proud, elegant Athenians of the North of earlier generations.

Now at last, the city has got a good new big building in its centre. For almost as long as anyone can remember, there was a large hole in the middle of the academic enclave, on the southern edge of the medieval Old Town, which runs down the moraine tail from the castle crag to the palace at the other end of the Royal Mile. The gap was roughly on the site of the south gate in the old city wall. Here, Chambers Street runs roughly east to west, and marks the boundary where the (largely revived) picturesque medieval and Georgian fabric changes to nineteenth-century eclecticism. At the west end of the street, four other roads meet, making the corner site one of the most important visually in the whole city.

The void has been filled by Benson + Forsyth's Museum of Scotland, an urbane and imaginative contribution to the cityscape. It takes cues partly from its neighbours and partly from history and the landscape. The new building has external walls of veined buff and slightly pink sawn Moray sandstone, which resonates with (though does not copy) the traditional Edinburgh ashlar (the old local quarries are worked out). In effect, the stone walls are a shell from within which the white rendered concrete core rises to a shallow concave roof. The stone takes its height from its neighbours on both sides, Victorian medieval to the south, and to the east along Chambers Street, the Royal Scottish Museum, a confident mid-nineteenth century building, designed in international revived Florentine style by Captain Fowke (of Albert Hall fame). Fowke's raked ashlar plinth is echoed in parts of the new building's base. The main impact of the new design does not come from contextualism: a stone cylinder at the corner links the two external walls and forms a landmark at the junction of the five roads. Though not a stair, it is reminiscent of the drum stairs of Edinburgh's tenements. Its smooth stone walls are incised and penetrated by slots and openings which are carefully adjusted to frame dramatic views over the city; the most important opening through the thick walls is the main entrance from Chambers Street.

Once across the bridge and inside, you are led up to the left along stone stairs or gentle ramp of the long thin entrance route. You are drawn on and upwards by the lines of raked ashlar, which has come in through the drum with you. The space is in part quite low, and seems a bit dark when you arrive, but its directionality is confirmed by a vivid splash of light at the end. Luminance spills over the reception desk which inflects you sideways. It comes from the great glazed triangular court, which is geometrically an extension of Fowke's wonderful glass and iron great hall, as light and elegant as the outside of his museum is pompous and stolid. When alterations to the nineteenth-century building are complete, there will be a luminous axis parallel to Chambers Street running through the whole complex. A huge window at the pointed west end increases the luminosity of the space and focuses on the Greyfriars Church, a late Gothic building which figured large in seventeenth-century Edinburgh history.

The triangular Hawthornden Court is the main orientation centre in a building which is supposed to tell the story of Scotland from earliest times to the present. To do this, time has been divided up by level, with natural history and anthropology in the basement, medieval and renaissance affairs on the same level as the orientation court, and the eighteenth, nineteenth and twentieth century collections on upper floors. There is a huge number of exhibits which vary from small and delicate objects like jewellery to large machines dating from the Industrial Revolution. This kind of museum is now rather unfashionable in museological circles, but very popular with the public. It is low on dioramas, interactive devices and massive quantities of explanatory matter, and the collections consist of objects brought together by generations of curators to form a sort of cultural cock-a-leekie, a rare Scots dish made to an unrepeatable recipe.

Each level is treated in a different way. The lowest floor with its ancient stones (Roman, Pictish and even earlier) is intended to be crypt-like, with graves cut into the rock, and the stones standing on its rough surface. But not only large and heavy objects are shown here. A tribe of rather alarming abstracted bronze humans by Eduardo Paotozzi (himself from Edinburgh) emerges out of the subterranean gloom to demonstrate early jewellery. There are dioramas of geology and natural history. (Benson + Forsyth were responsible only for providing the spaces down here and there is perceptible difference in quality from the floors above.)

On ground level is a depiction of the birth of the Scottish nation out of a melding of Angles, Vikings, Anglo-Normans and Celts under a much troubled monarchy. Many of the objects here are small and precious, and are displayed in niches cut into the thick walls of small rooms.

Above this are the Industrial Revolution and Empire floors, devoted to the great expansion in prosperity brought about when the Union gave impoverished Scotland access to English and world markets. Here are the big machines that provided the power, dominated by a Newcomen atmospheric steam engine, over two stories high set in the central nave which is lit by clerestories under the curved roof that is now revealed as a device for reflecting luminance downwards. Of course, there are both small and huge objects to see, and the machine hall is surrounded by galleries which offer a great variety of spaces for viewing smaller pieces as well as looking out over the void. square and round box galleries hover at each end of the hall as Euclidean solids in space. On the fifth level, they contain pottery and silver. On the top (sixth) level is the twentieth-century collection where the little round gallery is a cinema and the square one a memory bank (I suppose there had to be one somewhere). The collection has been assembled by asking the public, and prominent people from Tony Blair downwards, to suggest objects. The space was not finished when I visited it, but the collection did not look promising, ranging as it did from washing machines to a Scottish soft drink, Irn Bru. Again it is clear that Benson + Forsyth were not involved with the display, but apparently the whole affair is expected to last only five years.

Above, and reached by spiral stairs or lift is the first roof terrace, on which it is proposed to create a natural Scottish landscape. Finally there is the upper roof terrace, cradled as what the architects call a 'hanging valley' inside the curve of the roof. Up here, visual links with the .city are made most explicit. The stone entrance cylinder of the museum can be seen as a sort of echo of the Half Moon battery of the Castle. The dome of Edinburgh University's Old Quad at the other end of Chambers Street is framed in front of Arthur's Seat and Salisbury Crags. To the north is the sea, the Firth of Forth, partly obscured by the ragged skyline of the Old Town.

On descending through the different levels of the building, it becomes clear just how much thought has gone into making it. Alternative promenades architecturales offer each visitor an opportunity to make a personal route through the treasures and their spaces. Unexpected enfilade views have been created across galleries, linking volumes and visitors; little balconies and internal terraces allow you to reflect on the big spaces; light splashes from above and sideways with Soanian drama; great sympathy is shown for the objects, with the materials of cases chosen and detailed appropriately; the quotations from medieval buildings from Scarpa, Corbusier, Aalto and the rest are incorporated in an unobtrusive way, so that the different layers of the building resonate with the collections they hold. Above all, the museum has been designed with love for both collections and city: think for instance of the way in which, next to the place where the National Covenant is displayed, a little window looks out over Greyfriars Churchyard, the very spot where this key document in Scottish history was signed by patriots when, in 1638, Charles I tried to impose a brand of the English church on Scotland.

Walking along the street to the north, and turning round for a moment, you realize that the whole building both inside and out, is a contemporary example of a quality that has been remarkably out of fashion for a long time: the picturesque. The exterior's strong but sympathetic urban figure, and the matrix of places inside make the museum a proper contribution to one of the world's most picturesque cities. Although impossible to copy, this very popular building may mark a new phase in Edinburgh architecture. All who love the city hope so. P.D.

Architect

Benson + Forsyth, London

Project team

Gordon Benson, Alan Forsyth, Kevin Adams, John Cannon, Ian Carson, Eleanor de Zoysa, Catriona Hill, Annabelle Henderson, Jim Hutcheson, Debby Kuypers, Reza Schuster, Peter Wilson

Services engineer

Waterman Gore

Structural engineer

Anthony Hunt Associates

Lighting

Kevin Shaw Lighting Design; Butler and Young Photographs

Keith Hunter