Urban cubism: signposted by a glass cube, this museum complex revives a Stuttgart square.

Stuttgart's Kleine Schlossplatz (Small Palace Square) was built in 1968, in an age when motorist freedom was the dogma of urban planning. It is situated diagonally across from the Neues Schloss, one of Germany's largest Baroque palaces. Barren and uninviting, the square was never much more than a large raised concrete platform covering a motorway crossing and tram lines, and had been a depressing public and political eyesore for more than a quarter of a century.

During the 1980s, an investor-led competition put forward an urban design masterplan by I. M. Pei, but the local community did not vote in its favour. In 1993, Behnisch & Behnisch alleviated some of the grimness by introducing a generous flight of steps which won public acclaim. But the real breakthrough came in 1998 when the City of Stuttgart held an international design competition based on a mixed cultural and commercial use of the site. A staggering number of 431 architects submitted their proposals. Ultimately, Stuttgart-born (but now Berlin-based) architects Rainer Hascher and Sebastian Jehle won with a design that clearly separates the new art museum from any speculative commercial development.

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Today the rectilinear mixed-use office and retail building plays a secondary role compared with the glass-clad Art Cube, which acts as a striking landmark for the museum complex, a major intervention that has succeeded in miraculously revitalizing Stuttgart's city centre. Nothing but Behnisch's flight of steps remains as a reminder of the old urban condition. Yet Hascher Jehle's crystalline landmark is only the most visible part of the Museum--more exhibition spaces are housed in the former subterranean traffic tunnels as the architects skilfully and imaginatively utilize the remnants of the old infrastructure.

The building lies on Konigsstrasse, the main pedestrian zone, next to the imposing Neo-Classical arcades of the old Stock Exchange and opposite the Neues Schloss. The galleries at lower ground level occupy a 115m long section of the old tram tunnel, directly under the Kleine Schlossplatz. As you enter the museum from Konigsstrasse, the generously glazed 450sqm foyer is flooded with daylight, its separate shop and espresso bar bearing no trace of what was once the entrance to an infrastructural Hades.

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Exhibition spaces begin on the ground floor, behind a group of multifunctional rooms, and continue 60m down underground. You can peer into the open circulation area of the lower ground level directly below. Daylight filters through window strips high above, in the pavement of the Kleine Schlossplatz, helping to dispel any feelings of claustrophobia in the long corridors with their neutral whitewashed exhibition spaces. Nothing disturbs these calm, silent caverns of art, not even the noise and vibration of the 50 000 vehicles that rumble daily through the tunnels on both sides.

The 1100sqm gallery space above ground is accessed by a steel staircase that climbs up between the smooth outer glass facade and the inner reinforced-concrete cube, clad externally with rough limestone. The upward journey is an object lesson in how to hang a glass facade from the top while minimizing any obtrusive structural elements. Engineer Werner Sobek has performed a miracle of transparency, using giant glass panels measuring 4.10m X 2.50m, the largest possible in Germany. Each side of the cube has 56 panels and the homogeneous impression is further enhanced by the use of a smooth metal seal ensuring a continuous outer surface, so that viewed from an angle, the sheets of glass appear seamless. On the inside, 600mm thick vertical glass blades strengthen the facade against wind pressure. However, a degree of transparency had to be sacrificed for the sake of solar protection. Imprinted white horizontal lines (like blinds) on the outer glass filter the sun's intensity. So only 24 per cent of solar energy penetrates the interior, with triple glazed and argon-filled glass panes effectively dispelling most of the heat.

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Given the nature of the building with its precious works of art, it is imperative to maintain constant temperature and humidity. Inside the gallery spaces, 50 to 60 per cent of the heat gain is neutralized by cooling in the concrete ceilings. The remaining heat load is handled by conventional air conditioning which maintains an average temperature of 20 deg C and a humidity of around 50 per cent. Taking all these energy protection and saving measures into account, the Art Cube undercuts current German energy conservation rules by over a quarter--a highly respectable achievement for a construction that to all intents and purposes resembles a heat trap.

The crowning achievement of the glass cube is the restaurant on top of the three gallery floors. Were it not for the 114 moveable multifunctional louvres integrated within the glass roof as a combined shading, cooling and heating device, guests would be sitting right under the open sky. This fifth level restaurant floor offers the most spectacular panoramic views of the Stuttgart skyline. Kleine Schlossplatz has found a new home, 21 m above the old one, but Stuttgart's new social gathering space is an impressive and civilized change from its grim late '60s predecessor.