For more than 2,100 years the ancient Chinese built chang cheng, or long walls, in the north of their empire to defend it from nomadic invasion. Recording in earth, stone and bricks the protracted and violent conflict between the nomadic tribes of the north and the sedentary farming peoples of the south, these structures -- Great Walls of China -- go far beyond history. So labour intensive, and so time and material consuming were these projects that, even after the passage of centuries, long sections of the Great Wall remain as major landscape features, thus defining a special geography of north China. William Lindesay was lured to China in 1986 to explore the Ming-dynasty Great Wall, which evolved between 1368 and 1644 in direct response to the threat of a re-invasion by the Mongols. Travelling on foot in 1987 between its most westerly and easterly points, Lindesay discovered "that the Great Walls are not objects, but subjects." Guided in his belief that "field work is the key to understanding" he has remained in China for 15 years to systematically explore th