Or as the late U.S. President Richard Nixon put it: "It sure is a great wall."

The wall, which has a total length of over 3,500 miles (6,000 kilometres, is a monument to China's traditional fear of the barbarians to the north, and to the power and determination of many of its rulers.

Construction began in the fifth century BC when China was divided into a number of rival kingdoms. In the third century BC, the emperor Qin Shi Huang conquered all the other states, creating the first Chinese empire, and also joined up various sections of wall then in existrence along the northern borders to form the first Great Wall.

Re-buiIt, renovated and extended many times over 2,000 years, the Wall by the end of the 19th century stretched around one-twentieth of the world's circumference. The last major repairs were carried out during the Ming dynasty (1368-1644).

Most of the wall was about 25 feet high and 19 feet wide at the top, with about 25,000 towers about two arrow-shots apart so that the guards could cover its entire length, from the Yellow Sea to the Gobi Desert. The idea that the wall represents, of trying to enclose an entire country with a single man-made barrier, seems fantastic, and there is a continuing debate about whether the wall was in the end any use in keeping out the northern barbarians. Despite its existence, many groups of tribesmen over the centuries succeeded in breaking through and subjugating China, including the Mongols and the Manchus. But the wall did establish a border within which China's civilization could develop.

The section of the wall most usually visited is at Badaling to the north of Peking. A train was built up to the wall at this point in the early part of this century for the first tourists to allow their childen to be regarded as Dr Johnson described.