Stretching 6000km in a dotted line across China, the Great Wall was begun in the fifth century BC and was still being built up to the sixteenth century. Today's surviving sections, placed end to end, would link New York with Los Angeles, and if the bricks used to build it were made into a single wall 5m high and 1m thick, it would more than encircle the earth. Even at ground level, and along the small, most-visited section at Badaling, constantly overrun by Chinese and foreign tourists, Wan Li Changcheng (The Long Wall of Ten Thousand Li), is clearly the PRC's most spectacular sight.

The Chinese have walled their cities since earliest times and during the Warring States period, around the fifth century BC, simply extended the practice to separate rival territories. The Great Wall's origins lie in these fractured lines of fortifications and in the vision of Qin Shi Huang , who, unifying the empire in the third century BC, joined and extended the sections to form one continuous defence against barbarians. Under subsequent dynasties - the Han, Wei, Qi and Sui - the wall was maintained and, in response to shifting regional threats, grew and changed course. It did lose importance for a while, with Tang borders extending well to the north, then shrinking back under the Song, but with the emergence of the Ming it again became a priority, and military technicians worked on its reconstruction right through the fourteenth to the sixteenth century.

For much of its history, the wall was hated. Qin Shi Huang's wall, particularly, was a symbol of brutal tyranny - he wasted the country's wealth and worked thousands to death in building it. It is estimated that he mobilized nearly a million people to construct it, but other dynasties surpassed even that figure. Many of the labourers were criminals, but in the Sui dynasty, when there weren't enough men left for the massive project, widows were pressed into service. A Song-dynasty poem expresses a common sentiment:

The wall is so tall because it is stuffed with the bones of soldiers,
The wall is so deep because it is watered with the soldiers' blood.

The irony, of course, is that the seven-metre-high, seven-metre-thick wall, with its 25,000 battlements, did not work. Successive invasions crossed its defences (Genghis Khan is supposed to have merely bribed the sentries), and it was in any case of little use against the sea powers of Japan and later Europe. But the wall did have significant functions. It allowed the swift passage through the empire of both troops and goods - there is room for five horses abreast most of the way - and, perhaps as important, it restricted the movement of the nomadic peoples in the distant, non-Han minority regions.

During the Qing dynasty, the Manchus let the wall fall into disrepair as it had proved no obstacle to their invasion. Slowly the wall crumbled away, useful only as a source of building material. Now, though, the Great Wall, as Nixon might have added, is great business. At the restored sections, Badaling , and to a lesser extent, Mutianyu , the wall is daily besieged by masses of visitors. Distant Simatai , Jiugulou and Jinshanling are much less crowded, and far more beautiful. To see the wall in all its crumbly glory, head out to Huanghua , as yet untouched by development. Other places to see the wall are at Shanhaiguan, Zhangye and Jiayuguan.