Ypres in 1200 was a textile city. Flemish weavers in this part of the Low Countries made the finest woollen cloth in Europe, traded fairs at Champagne and warehouses in London, and the Cloth Hall was the heart of that trade - market floor, warehouse, civic hall and watchtower in one. The belfry went up first, beginning around 1200; the long market hall followed from about 1230; the whole ensemble was finished in 1304. Because almost the entire building went up in a single push in the 13th century, it shows no architectural patchwork - it is one continuous statement in pure Gothic. You could once tie a boat up to its back door: the Ieperlee canal ran along the rear, bringing wool in and cloth out by water. The canal is now covered, the cloth trade long gone, but the building stands as the surviving monument to a moment when Flanders was the workshop of Europe.

When the First World War caught Ypres on three sides, the Cloth Hall was the largest target in the salient. German artillery on the surrounding ridges shelled the town day and night for almost four years. The hall did not fall all at once. Photographs from 1914 show the belfry already wounded, smoke rising from the gables; by 1915 the great Gothic windows were empty sockets; by 1916 the roof was gone and the long market floor lay open to weather. The destruction continued through the war. In photographs from 1918, the Cloth Hall is a ridge of brick and rubble with the belfry stump rising from it, less a building than a small mountain. Painters and photographers from many countries made it their image of the war - the ruin became more famous than the building had ever been. A 1918 painting of the Cloth Hall in ruins by the Scottish artist James Kerr-Lawson hangs today in the Canadian Senate Chamber in Ottawa.

After the war there was a serious argument that the Cloth Hall should stay a ruin. Winston Churchill wanted Ypres preserved as a mausoleum to British arms; the Belgian government in 1919 was actively considering two plans that would leave the Cloth Hall and the cathedral as permanent ruins, perhaps surrounded by a screen of trees. But the people of Ypres wanted their town and the architects, led by Jules Coomans, made a different choice. The new Cloth Hall would not be a reinterpretation, a tribute, or a modern building referencing the old one. It would be the same building. Surveys made before the war, photographs, drawings, the salvaged elements from the rubble itself - everything that could be reused was reused. The western end and the base of the belfry still contain original 13th-century masonry. Most of what surrounds them is a faithful copy, indistinguishable to anyone who is not measuring.

Work on the Cathedral of St Martin behind the Cloth Hall finished first; the Cloth Hall itself was rebuilt in stages between 1933 and 1967 - thirty-four years of patient work that outlasted another world war, in which Ypres was occupied for four more years. The reconstruction had to interpret what had been there before with deep humility. The niches across the facade had once held life-size statues of the Counts and Countesses of Flanders. After the war most of those niches were left empty, because the original statues were gone and no one wanted to fake them. The central niches, though, were filled with new figures: Count Baldwin IX of Flanders and his wife Mary of Champagne, the legendary founders of the building; and King Albert I and Queen Elisabeth, the Belgian royals under whose reign the rebuilding began. Between them, above the great central archway called the Donkerpoort, stands Our Lady of Thuyne, the patroness of Ypres.

The belfry above the Cloth Hall carries a carillon of 49 bells, a watchtower's view of the town, and a centuries-old custom that is now performed only in jest. In less enlightened times the people of Ypres threw live cats off the belfry. No one is quite sure why - possibly because cats were associated with the devil, possibly because the town kept too many cats to guard the wool against mice and had to cull them somehow. The Kattenstoet, the Cat Parade, takes place every three years in May and culminates in a jester throwing stuffed toy cats from the belfry into the crowd below, who try to catch them. It is the kind of half-embarrassed, half-delighted ritual that Belgian cities do better than most. Cat Parade or no, the belfry is the dominant landmark of the salient. Soldiers in the trenches in 1916 could see what was left of it from miles away. So could the German gunners.

In 1999, UNESCO inscribed the Belfries of Belgium and France on the World Heritage List - a group of municipal towers recognised for their architecture and for what they represented: the rise of secular civic power in medieval Europe. The Ypres Cloth Hall belfry is on that list, despite being almost entirely 20th-century. UNESCO's decision is itself a small statement about authenticity: the building is what the building was, rebuilt by the descendants of the people who built it the first time, from the same plans and largely the same materials, in the same place, for the same civic reason. Inside, the upper floor of the Cloth Hall now houses the In Flanders Fields Museum, named for John McCrae's 1915 poem. The building that the war destroyed now exists to explain the war that destroyed it. There may be no more honest way for a building to come back from the dead.