I've seen a few city walls before; the remnants of London's, York's, Dubrovnik's etc. But the ones that surround the old centre of Xi'an are really something else, for a start, unlike most city walls in China, (including Beijing's which were knocked down many years ago) they've survived nearly 700 yrs of dynastic power shifting, without much need for restoration. They are truly massive - a cross section of 12m wide at the top and about the same height and I reckon even the largest cannon ball would glance off. Hell, they could probably withstand a cruise missile. Forming a rectangle about 4km x 3km enclosing the old city, and with beautiful defensive towers along each edge, they also make for a great cycle ride. I chose to do it at dusk and during the eighty minutes it took to ride the complete circuit the sun went down and the lanterns and lights came up to reveal the watch towers in all their splendour.
Xi'an for centuries marked the end of the silk road, and as such was the nexus of Chinese import and export before the sea routes became explored, charted and secured. It's truly amazing what can be built with a vast army of indentured labourers and the wealth of cannily exploited natural resources sold to the global market, modern Dubai for instance has clearly heeded a few of these lessons.