At the top of the steps we lean against the cool stone of the battlements and look back. The Great Wall tumbles and climbs its way over the mountain ridges, just as it does in all the pictures, disappearing at last into the blue distance. Down on the plain to the south we can make out a tiny patchwork of fields and the dots of houses. The plain to the north is just a silvery-green blur glimpsed here and there beyond further hills. The breeze is cool but the sun is warm, and on the slopes below us the plum trees are white with blossom. And of course the real lines of defence are now elsewhere - in remote compounds, with high perimeter fences and missiles in buried silos. The amiable clown who wanders the battlements in Manchu warrior garb, crying 'boo' and waving his sword to startle the tourists, is theme-park 'tradition' at its emptiest.

But the Wall remains none the less a potent image of war. We can still imagine the ancient stir of fear and excitement here on the steps when puffs of smoke from some distant watch tower signalled the approach of the enemy -Tamburlaine, perhaps, or Genghis. My son - his mother is Chinese - has just recalled that this is the only human construct visible from the moon. Sad mark of a divided world. One's particular perception - of these stones, these blossoms, these jagged mountains - seems to figure the ...