Pax Romana: Richard Meier triumphs in Rome, creating a new shelter and museum for Ara Pacis.

As you stroll down the via di Ripetta from the medieval centre of Rome towards the Piazza del Popolo and the original main gate of the city, the genial succession of Baroque and Neo-Classical masonry facades is interrupted by a simple white rectilinear form that gradually reveals itself to be the concrete portico of an entirely new building. It is the entrance to Richard Meier's Ara Pacis museum--the only major contribution to the fabric of the city within the Aurelian walls for over 60 years.

Naturally, it has been immensely controversial. Many were horrified that a new building--and a big one at that--was to be allowed at all. Some resented the fact that there was no design competition and the mayor had personally chosen Meier rather than someone else--preferably an Italian. Among other demonstrations against the building, Vittorio Sgarbi, a well-known art critic, burnt a model of it next to the site. He called it 'an indecent cesspit by a useless architect'; others likened it to a petrol station or a pizza parlour. But something had to be done to save one of the most moving, revealing and best preserved of all the monuments of the imperial era.

The Altar of Peace was built between 13 and 9 BC by Augustus, the first emperor, following victories in Gaul and Spain. Originally on a nearby site, it had been smashed into hundreds of bits, many of the largest of which were in foreign museums. In Mussolini's time, in an astonishing display of archaeological and conservation skills, the monument was very largely re-made, using Roman fragments and copies of pieces held elsewhere. The original site was not available (much of it being taken up by a church) so a new one was chosen between the mausoleum of Augustus and the Tiber. In 1938, Vittorio Ballio Morpurgo built an enclosing structure to keep off the rain.

Originally, the monument was in the open air. The altar at which the sacrificial animals were slaughtered was surrounded by a stone enclosure. Two friezes decorate the exterior walls. The lower one is an elegant composition of acanthus leaves, but it is the higher one that is a marvel of the ancient world. Carved in fine Carrara marble, probably by Greek craftsmen, are reliefs of Augustus, his family and aristocratic friends, all wearing his newly prescribed ceremonial togas and going in procession towards the altar's opening ceremony.

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You know them: there is the emperor, his wife Livia; there, the (then) heir Marcus Agrippa and even little grandchildren clutching their parents' legs for security. All are individual likenesses, dignified yet animated by their involvement in the procession. How peaceful and companionable they all seem--yet many were murdered, and some of the murderers are to be seen in the company. Is it ironical that at sacrificial ceremonies, the whole place must have stunk of blood?

In a curious way, barbarian destruction of the altar may have helped to preserve it, for fragments buried in the ground were kept out of the weather but, once recreated, the altar had to be saved from erosion, so Morpurgo's big shed was vital. By the 1980s, however, it was clearly inadequate, exposing the altar to drips, and pollution and vibration from heavy traffic on the Tiber embankment. A whole new shelter was needed to provide an atmosphere acceptable to modern conservators.

Its context is strange. Mussolini brutally created an urban square round the tomb of Augustus--then, as now, a circular mound ringed by cypresses, overgrown with lank grass and infested by mangy cats. (Il Duce had plans to be interred there himself, in one of the niches vacated by long scattered imperial ashes--given time, he would doubtless have tidied the place up). Three sides of the Piazza Augusto Imperatore are in fascist Neo-Classicism: heavy brickwork relieved by travertine dressings. The west end of the square was formed by Morpurgo's building hard up against the river embankment. So Meier's museum had the tricky tasks of fitting onto a long thin site, working round the altar which could not be moved, forming the fourth side of the stodgy square, providing a suitable setting for the altar's majesty and, as far as possible, exposing it to external view.

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Meier has succeeded triumphantly. From the piazza side, his building is approached past a cascade and pool, then up gentle flights of steps to a terrace at the level of the embankment road. Onto the terrace, a great wall clad in riven local travertine slabs thrusts from the building, slightly inflected to draw you in into the portico. This is formed by the conjunction of the warm rough travertine and an angle of beautifully cast white concrete that rises to form the other side of the entrance, turns horizontal to become the roof of the entrance hall, then projects over the stone wall which, in contrast to the smooth whiteness of the concrete, is naturally and elaborately decorated with fossil plants and shells.

In the entrance hall, the travertine to the left is flooded with luminance from rooflights, dramatically bringing out the patterns of the fossils and finally falling on a row of classical heads. Yet after the brightness of the Roman sky, the space seems almost sepulchral. A line of seven circular concrete columns finished in white waxed marble plaster runs in front of the white right-hand wall, creating a zone for reception desks and simultaneously drawing you forward to the main hall. Here, the Ara Pacis sits in the centre of a high luminous gallery with long glass walls overlooking the embankment to the west and the mausoleum on the other side. Supported on four concrete columns, the gridded roof modulates the sky's light. Apparently, the ancient structure is flooded with natural daylight. In fact, the light is much reduced in intensity by greyish low-e glazing and external horizontal louvres of translucent glass. It becomes clear that the darkness of the entrance space is an ingenious tactic, for its relative gloom persuades your eyes that you are in ordinary daylight again when you get to the great hall, particularly when morning and evening sun seem to shine without being modified through the glass walls.

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After the wonderful object itself, the rest of the building is inevitably a bit of an anti-climax. Behind the altar is a void with stairs leading down to the basement and the lower level of the double-height lecture theatre. Meier refuses to talk about the rest of the basement, which contains offices, temporary exhibition space and a digital library covering all things Augustan--their interior design was by another hand. Meier's terrace at roof level with its bar and cafeteria offers previously unattainable views of the city. On one side is the river's tree-lined embankment (where, the city says, the endless traffic will one day be taken underground); on the other side is the mausoleum (nearly as difficult to interpret from up here as it is from street level).

The terrace is a good place to brood over Meier's contribution to the fabric of Rome. Though the museum is a big--or at least long--building, forming the whole side of the square, its massing is broken up, so it seems to be much more part of the haphazard nature of Baroque Rome than Mussolini's stereotyped Neo-Classicism. It grows out of its site. The only thing to be preserved from Morpurgo's time is his travertine plinth on the east side, which is inscribed with the Res Gestae (the autobiographical Deeds of the Divine Augustus) in beautiful Trajanic lettering. To the west, Meier's low travertine wall on the site boundary traces the ancient curve of the Tiber bank. And the building allows the altar to take its place in the city for the first time in some 16 centuries. You can see it from the embankment, particularly at night when, lit up inside the gallery, the aristocrats of ancient Rome stare arrogantly out from the past into the present and future.

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