Diocesan dialogue: Peter Zumthor orchestrates an angelic conversation between old and new.

Cologne must have one of the highest ratios of museums to population of any city in the world. From the brilliant Romisch-Germanisches museum (which celebrates the life of the colony founded by emperor Claudius) to the museum of chocolate (which celebrates elegant gluttony), from the collection devoted to German sport to the one about eau de Cologne, there can scarcely be an aspect of human culture not examined. The latest, the Diocesan, displays the magnificent art collections Germany's richest bishopric has built up over 1000 years.

Peter Zumthor won the competition for the new building in 1997. He was faced with a delicate and complex site, the ruins of a great church. At the end of the War, the medieval heart of Cologne was bombed flat, leaving the greatest cathedral in Germany towering over rubble. With immense determination, the city rebuilt its centre largely following the medieval street pattern, though with added urban motorways and twentieth-century buildings. Many shattered monuments were restored, but the greatest and richest medieval church, St Kolumba's, remained a fragmented shell enclosing a peaceful garden. In the 1950s, Gottfried Bohm built a small chapel on the site to house a statue of the Virgin that had miraculously survived amid wholesale destruction. It was not one of Bohm's most memorable efforts--an octagonal Basil Spenceish, Gothic-Moderne affair, lacking the amazing expressionist invention of the architect's '60s buildings like Bensberg town hall and Neviges Pilgrimage Church. The Cologne chapel was liked but excavations in the '70s revealed the origins of the Gothic church, yet destroyed the garden.

Zumthor's task was to display the excavated ruins and preserve the Bohm chapel, yet to cram enough gallery accommodation onto the site to display the extensive diocesan collections. At the same time, all historic elements had to be preserved and, where possible, enhanced. He responded by throwing a high concrete ceiling over the excavated ruins and the chapel, then arranging gallery accommodation above and around it, an obvious strategy perhaps, but fraught with problems. Many people objected to the chapel being ceiled over (including Bohm, even though at one point he had suggested a similar stratagem). The ruins were to be disturbed as little as possible, so the ravelled multi-level warren of history had to be analysed in great detail to determine where supports could be located. To preserve the excavated remains, they had to be kept at outdoor temperature and humidity while the works in the collections had to be kept within very strict curatorial limits.

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So great was the pressure for space that fragments of Gothic walls had to be incorporated into the perimeter, causing complicated structural problems. As in most of Zumthor's buildings, the exterior reveals little. At ground level, it is pierced only twice: with the glazed entrance to the foyer of the main building and the separate opening for the chapel. Massive and fortress-like, the walls are made of the longest and thinnest bricks I have ever seen in a modern building; they are all some 36mm thick but vary greatly in length. Specially made in Denmark, they were burnt in charcoal kilns to produce gentle variations on honey-coloured pale grey. Craftsmanship is immaculate, with mortar of nearly the same colour as the bricks and very thick horizontal joints like Roman masonry. Pointing is almost but not quite flush, with the mortar very slightly recessed, producing a soft, almost textile-like surface that demands to be stroked--strange in massive load-bearing structures 600mm thick.

At first floor level, the nature of the brickwork changes, with large passages patterned by random perforations. The impervious texture of the lower wall is transformed burka-like into a veil of the same material. Above, the masonry's impervious texture continues, now alleviated by the great panes of the top floor windows. These have slightly projecting metal frames, like those on the red brick walls of Lewerentz's Klippan church, a masterpiece of masonry, light and shadow that is recalled more than once by the diocesan museum. The museum's roof-line follows the different volumes of the galleries and is blunt and sharp against the sky. While the building is introverted and somewhat fortress-like, it is not out of scale with its surroundings. The only exception to the general rule of small-scale mediocrity is the restored smooth lightness of Bruno Paul's 1928 Mendelsohn-like Dischhaus that contrasts with the museum's blocky massiveness.

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Entering, you are faced with a blank wall, which deflects you left towards the reception desk. Here, everything is flooded in daylight from the foyer's window wall, which overlooks the court that Zumthor has created to recall the old churchyard. Young trees rise from a gently rounded form covered in grey gravel. The museum's grey brickwork continues and forms one wall of the court, to the right are stone medieval fragments, carefully preserved with their repairs from different periods made quite obvious. The other two walls of the court are formed in concrete, rough poured to show its aggregates as almost geological strata like the concrete at Zumthor's Bruder Klaus chapel at Wachendorf.

Returning to the foyer, the ruin hall beckons. A 12 metre high floor-to-ceiling opening takes up most of the end wall of the foyer. To keep the two climates apart are full-height leather curtains, reminiscent of the leather valances that contain the hot rooms in the thermal baths at Vals (AR August 1997). But there, the curtains are sensual, black and slippery with condensed steam. In Cologne, they are chaste, dry and warm brown. The change in luminance between the museum's bright foyer and the mysterious twilight over the ruins in the great space is dramatic. At first, I thought that there was a representation of a starry night sky above the ruins, but once my eyes had adjusted to the dimmer space, it became clear that these manifold points of light are the result of the band of porous lacy brickwork that makes the outside so strange. It allows daylight, outside air and street sounds into the great space. Light changes minute by minute as sun and clouds move; it is reflected off the ceiling and sometimes has a greenish tinge from the leaves of surrounding trees. On sunny days, needle-like shafts of sunlight suddenly illuminate a Roman cellar or a line of modern concrete columns. How are birds kept out?

You are conducted over the ruins on a zig-zag wooden bridge, which has a rail that is both easy to hold and welcoming to lean on. Below, illuminated by conical semi-industrial lamps as well as the patterned daylight, are the crumbled brick and stone walls, arches, vaults and column bases of nearly two millennia of civilisation: Roman, Frankish, Carolingian, Romanesque, early and late Gothic piled indifferently on top of each other.

The bridge leads to the sacristy, now roofless, a small broken-vaulted medieval court containing Richard Serra's rusted steel The Drowned and the Saved (1992-97) erected over human remains found in the excavations. Returning along the bridge, you become aware of strange soft music in addition to muted street noises. Pigeon Soundings by Bill Fontana takes, mixes and abstracts the sounds of the pigeons that used to flock on the site. As he did at Vals and the Swiss pavilion at Hanover (AR September 2000), Zumthor is trying to involve senses other than sight and touch. Surely scent will soon be included, for the ruins are still consecrated and services (presumably including incense) can be held in the great space.

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From the numinous atmosphere of the ruin hall, you return to the foyer to take stairs to the first floor. A couple of short flights and landings take you to a long straight run of steps lit from the top between parallel walls like a medieval flight in the middle of a castle or Libeskind's long stairs at the Jewish Museum in Berlin (AR April 1999). On the first floor, galleries are windowless, so intensely demonstrate the museum's hanging policy, which must be unique. Pictures are displayed with no attribution--though you do get a handlist. They have no obvious organisational scheme, so for instance an early Renaissance wooden figure of Christ in torment can be found next to a couple of Warhols (there is little in the collections between the sixteenth and the twentieth centuries). The aim is to stimulate contemplation and to open new perspectives. The most intense space on this floor is the treasury, where silver medieval reliquaries and crosses enriched with ivory, enamel and glittering jewels are exhibited in vitrines, side by side with richly illuminated books of hours. In this dark room, light comes only from the displays, and distances between them are so small that you are virtually forced to examine the delicate work intently: your nose is inches away.

Another straight flight leads to the top level. Much larger than the one below because it covers the ruin hall, the floor is laid out like a medieval town centre with a linked series of central spaces fringed by individual galleries like houses round a marketplace. Up here, floor-to-ceiling windows generate pools of light, encouraging oblique visual axes across the central spaces, sometimes bringing the cathedral's magnificent nineteenth-century Neo-Gothic spires into play with paintings like the vibrant yellow Homage to the Square by Joseph Albers. As on the first floor, walls are of plaster which, like the concrete of the Bregenz Kunsthaus (AR December 1997), offers no obvious means of mounting the pictures: each must be separately supported by drilling into the wall, and when there are changes in the hang in future, signs of previous arrangements will be apparent, patinas of the past echoing in a small way how traces of the previous are to be found in different ways throughout the building. The massive walls are partly made of hollow bricks to provide thermal insulation. To reduce thermal movement, they are kept at constant temperature by circulating water drawn from an aquifer 70m below ground, modified according to season. Air is drawn from the ruin hall heated or cooled and delivered to the galleries through their ceilings; exhaust is via a continuous slot round floor edges.

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The central spaces are floored in grey terrazzo while the smaller surrounding galleries have floated concrete floors set about an inch above the terrazzo, a gentle hint of threshold that reminds you, sometimes with a slight stumble, that you are entering a special space. In one of these is the diocese's specially commissioned Joseph Beuys--all self-respecting German museums of modern art have to have a Beuys. In this case, it is not the often repeated trousers but a hatstand with overcoat and trilby dimly reflected in a golden wall. Of these smaller spaces, one of the most memorable is the reading room. Save for the window wall overlooking the city, it is lined with strongly figured veneers that create a calm, quiet sensation of being in the library of an urban club.

Rarely does a building of this size and purpose offer so many sensations, as you are reminded when you leave and find the stainless-steel door handles are bound in spirals of fine wire. It is the museum's final caress, and a reminder of Zumthor's commitment to architecture that moves all the senses. But as often with a new Zumthor building, Kolumba has created much controversy. At the museum's blessing ceremony, Joachim Meisner, cardinal archbishop of Cologne, and in effect the client, proclaimed that 'whenever culture is separated from the worship of God, the cult atrophies in ritualism and culture becomes degenerate'. That word, entartete, set off immediate stridently hostile reactions in Germany, where the Nazis' 1937 Degenerate Art exhibition notoriously attempted to destroy Modernism. Yet the cardinal can scarcely have been attacking his own museum, for it shows more clearly and movingly than almost any other contemporary building the continuity of Christian faith, and the way in which it is built on conjunctions and conversations between ancient and modern, particular and universal, temporal and spiritual.

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