WEALTH created the city wall of Pingyao more than 600 years ago. Poverty saved it from destruction by marauding Red Guards in the 1960s. Now the towering four-mile fortification that encircles the ancient town is in danger of tumbling down through neglect. The wall may be the most perfectly preserved in China, which half a century ago could boast 300 city walls. All but four have crumbled, razed by invading Japanese armies during the Second World War, by Chairman Mao’s zeal to do away with the old, and by economic modernisation. The other three city walls survive only in sections or jostle for space beside encroaching factory chimneys and faceless blocks of flats. Pingyao’s wall stands proud, as it did six centuries ago. A moat snakes around the base of walls ten metres tall and made of packed earth encased in a crust of dark grey bricks.