Architecture: Norman's Berlin conquest

The big glass lantern on top of the Reichstag sparkles with mirrored glass by day and is strobed by light at night, a sign that Berlin is once again the capital of a unified Germany. The public can even go inside this lighthouse to get the most spectacular panoramic views of the city from the double-helix ramps that spiral around its 40-metre diameter.

But the Reichstag renovation, by British architect Sir Norman Foster, is more than a magnificent cupola on top of an ungainly pile of stone built in 1898. Foster has boldly cut out the heart of the building, from roof to ground floor, to let the light beam down deep into the plenary chamber at its core. This surgery not only excises a lot of the past, it also allows the workings of the Bundestag to be transparently visible. Wherever you are in the building, you can look down into the debating chamber. Above the MPs is the sky. So keen was Foster to express the new age of enlightenment that he inverted a cone of mirrored glass below this lantern to diffuse and refract natural light deep into the cavernous building.

Foster explained the complicated geometry he used for the Duxford air museum as "a doughnut". At the Reichstag, a bagel might be a more apt word to describe his plan. Imagine a bagel sliced into three floors for administrative offices and press lobbies. Then put a cloche on top of the hole in the middle and stand the whole thing on the saucer-like debating chamber, and you can see how he has opened out an inward-looking building from another century. Below that debating chamber in the basement is a power station which runs on rapeseed oil - which gives some idea of the cavernous size of the Reichstag. The thermal station will power not only the Reichstag but the ministries and residences, designed by Axel Schultes and Charlotte Frank, which are being developed on an east-west axis behind it.

So much for a 19th-century imperialistic building facing up to the needs of a reunified Germany in the 21st century. But how to deal with the legacy of the 20th century? This is what troubles architects and art historians in the building site that is modern Berlin. All that remains of the Wall that divided East from West until 1989 are zigzagging red lines to show where it once stood, painted on the tarmac of the new roads and intersected by yellow traffic markings. The once wide open spaces of No Man's Land and Potsdamer Platz now support buildings by an international, star-studded cast - IM Pei, Frank Gehry, Philip Johnson, Arata Isozaki, Jean Nouvel, Renzo Piano, Richard Rogers, Aldo Rossi - all jostling for attention beside the historic buildings from the days of the kaisers.

Without turning Berlin into one big memorial site, history has to be respected and replayed alongside all the reinvention. There have been tremendous rows over the best way to do this. Simply removing all trace of the Nazi past is not a solution. In Berlin today there is no policy on how to preserve the past. Hitler's bunker, for example, is unmarked, although you can take the "Infamous Third Reich" tour, offered by one enterprising city company, Original Walking Tours. Sometimes history is told through archival photographs on the hoardings as the building or landscape gets a new role. All along Linden Lusgarten, which is being landscaped in a formal 18th-century manner, sepia photographs show past Berliners' use of the gardens - including the time it was a rallying point for crowds massed under the Nazi swastika.

Opposite, the old copper-clad Soviet block which was headquarters of the East German government and doubled as a bowling alley has been given a stay of execution and is having its asbestos panelling ripped out while everyone decides what to do with it. Where the Nazis burnt books in May 1933 in Bebelplatz there is a toughened glass panel inset deep into the square by the Israeli artist Micha Ullman, so that you look down into empty white bookshelves big enough to hold the 20,000 books burnt there.

Foster does not overlook the central role of the Reichstag in the history of Germany, which is why he leaves fragments from its past in place, such as the Russian graffiti from the Red Army occupation on 2 May 1945, when the Red flag flew above the Reichstag. Less obviously - but with more emotional impact - he has installed sculptures and paintings throughout the Reichstag that tell the narrative of 20th-century history. Positioned carefully, sometimes even poignantly, these artworks trigger a response that is sometimes joyful, sometimes spiritual. For many months Foster worked with specially commissioned German, and Russian, French and American artists - chosen to represent the four Allied powers who administered the divided Berlin the in the Cold War era.

German MPs who moved from Bonn for the first sitting today are going to get a few shocks. Such as the two bombs with a detonator on a table outside their cafe. Tisch Agregate by the late, great German avant-garde sculptor Joseph Beuys is just one of the artworks installed by Foster to make the experience of the space an unsettling one. He uses their artwork to tell the narrative of German history from its imperialist past (Reich means empire) to German reunification, with the Third Reich period provoking the most unsettling spaces of all.

In the MPs' lounge, Memorial to the MPs killed in the War, by Katherina Sieverding, consists of three books on three lecterns featuring photographs of the MPs taken out by Hitler when the Reichstag was set on fire in 1933. Today, politicians emerging from the underground link between residential blocks and ministries (still under construction behind the Reichstag) are confronted by a wall of bronze boxes housing relics of MPs who opposed Hitler within the Reichstag in 1933. The boxes are welded together by artist Christian Boltanski and titled Memorial to 500 Politicians.

MPs are singled out for the hardest-hitting reminders. Public spaces, such as the courtyards that house gigantic stone doormats by Ulrich Ruckrheim, or the big lobby that faces the road, are lit at night so that you can see the pair of Georg Baselitz shocking-pink paintings on either side of the room, which themselves are intriguing rather than unsettling. In the north entrance, American artist Jenny Holzer's four-sided column of red ticker-tape plays phrases from historical speeches given in the Reichstag for 12 days without repeating itself. The soundtrack gives onlookers a turn when it jumps tape and slips. "Every time there is a round of applause on the archival recordings she lets the transcript jump and drop a little so that anyone travelling in the lift looking out through the glass on to the column would think the cables had been cut," says Foster, who installed the work with Holzer.

This use of art makes the Norman conquest of the Reichstag far more important than the addition of the dazzling glass dome - which is the only visible sign of change from the outside. He likes to call this cupola a lantern because it beams light such long distances. The rays pass through an inverted cone-covered in mirrors to radiate and sparkle like an Art Deco mirror ball.

In working out his design, Foster built a one-twentieth sized model of the lantern and rigged it up on the roof. Then he went up in a bucket and worked out exactly how the light would fall - "Not just background light but to get a shaft of light right into the space". At 2am before the opening he was up on the roof lowering the levels and the intensity of the artificial light. For special occasions, Zenon lights will strobe the air.

A bore hole sunk 400 metres into the earth takes surplus heat and stores it, with the surrounding rock acting as an insulator to release it during winter. In summer, photo-voltaic cells on the roof capture the sun's heat to store it and power an electronic tracking device that shields the glass dome from the direct rays, preventing it from becoming too hot or too bright.

As important as revealing the workings of parliament was the cosmetic surgery needed for a democracy in the 21st century. The unloved and unlovely Reichstag is a little over 100 years old, although it looks much older. It didn't wear well, either. Paul Wallot won a competition in 1871 to build a parliament for the Reich, but it took 15 years for Wallot's design to get off the drawing board and on to the site of a razed palace near the Brandenburg Gate. When Kaiser Wilhelm II laid the last stone 10 years later, the project had swallowed 26m marks. This time round, the Bundestag insisted that the project would take no more than four years, and come in on time and on budget, for DM600m (pounds 210m), making the annual contribution by each citizen of Germany just DM2, or less than pounds 1 each.

Until 1954, it wasn't certain whether the Reichstag should be reconstructed or pulled down, but that year, reconstruction started. Since Berlin was no longer the capital, and the parliament had moved to Bonn, the building was used to house the German Historical Institute.

Foster was a joint winner with two others in a competition in 1991 to convert the Reichstag back into a parliament. His original design was not used, and instead Foster was asked to modernise an imperialistic building and take on board all its cultural and emotional baggage. Foster, who is inordinately proud of the project, calls it "a labour of love".

On a tour of his new building, Foster leads the press in his slipstream along the double-helix ramp. Determinedly fit as a marathon runner, he never lets up. And to mark his achievement, Foster will receive the prestigious Pritzker prize for architecture at the Altes museum on 7 June, six days after his 64th birthday.

London Pride

Anyone wanting to understand buzzwords bandied about by architects and Government decision makers should watch `London Pride', BBC2, 21 April at 11.20pm. Rather than carve up the countryside for more housing, the words that sum up current thinking are `brownfield sites" and "urban regeneration".