You mean that over-priced joint-venture wine produced on the mainland? No, I'm referring to what it was named after - China's 6,000-kilometre wall, the only man-made structure identifiable on satellite photographs of the Earth. Yes, a true wonder of the world. Indeed. Thousands of years after its construction it is still considered one of mankind's greatest feats of engineering. Why was it built? To keep the Mongols out of China, or at least delay their arrival and allow time for the home guard to be called up. How high is it? Its height varies, usually from six to eight metres. The wall was always built on ridges or the highest ground available to make it as inaccessible as possible from below. In parts its base was built a sturdy eight metres thick, tapering to five at the top. When? Construction was a long process, rooted in the joining of walled cities in the Warring States period (457BC-221BC), but stretching mainly through the Ming dynasty (1368-1644). Who? Hundreds of thousands of Chinese soldiers, labourers and prisoners. Where does it start and end? It stretches along China's north flank, from near Dunhuang in Qinghai Province in the west, to Shanhaiguan in Hebei in the east, where it hits the coast not far from Beijing. Was it effective? Certainly. Special beacon towers were placed at intervals of about 18km, from which drum or fire signals warned of invasion. Messages calling for reinforcements could be relayed along the wall and seen from strategic inland military posts. The top is wide enough along its entire length to accommodate five mounted cavalrymen. Were any other structures built into the wall? About 25,000 watchtowers, gates, fortresses, storage rooms and temples were incorporated. Were gates made for Chinese forces to pass through? That was deemed too dangerous in case of enemy occupation, so maze-like passes were built at strategic points. What is the wall made of? Fired clay, stone and earth. Were the same materials used throughout? Not entirely. That would have been difficult geologically, given its length. But the same stone, fired clay and earth were used as often as possible for the exterior, which was always built first. The rock-and-earth filler was more heterogeneous. Couldn't the rocks and earth inside shift and cause the outer wall to give way? No. The filler was compacted and the outer wall's comparatively narrow top and wide base helped to contain it perfectly, hence the 'great engineering' accolades. Today, a similar construction would probably have inner reinforcements and foundations below ground.