The big idea
The Great Wall is a long, heavy barrier that works with the land. It keeps to the crests so attackers must climb, stands on a wide footing, and is a tough shell of stone and brick around a heavy core of rammed earth and rubble. Its top sheds rain through spouts, and its towers and beacons turn a line of stone into a system that could watch and send warning fast.
Memory trick: “Follow the ridge, face the fill, drain the top, watch and warn.”
A bit of history: Walls were built in northern China from the Warring States period onward; the first Qin emperor joined and extended them from about 214 B.C., and the Han dynasty carried them far to the west. The Ming dynasty (1368 to 1644) built most of the brick and stone wall seen today, especially in the 1500s, to guard against raids from the steppe to the north. Badaling was built in 1505 and repaired later. At Mutianyu the wall was rebuilt from 1568, and from 1569 Tan Lun and Qi Jiguang added hollow watchtowers along the frontier near Beijing. Dates for particular sections vary, because walls were repaired and rebuilt many times.
Watchmen on a wall turn stone into a warning system. In Isaiah the Lord says: I have set watchmen upon thy walls, O Jerusalem, which shall never hold their peace day nor night
(Isaiah 62:6, KJV)