That action, which led to the downfall of the Míng dynasty, occurred at Shānhǎi, one of the three major routes through this location.
A demilitarized zone extending south from this location was established by the Tánggū Truce. Troops fleeing the battle of Rèhé fell back to this location, defended in the ensuing Operation Nekka. One of the stupidest decisions undertaken at this site was apocryphally blamed on the concubine-theft of Chén Yuányuán, which prompted an enraged Wú Sānguì to request Dorgon's help in killing Lǐ Zìchéng. That action, which led to the downfall of the Míng dynasty, occurred at Shānhǎi, one of the three major routes through this location. Wáng Zhèn was executed by subordinates for stupidity after leading a march between two fortresses in this site, allowing angry Oirats to capture the Zhèngtǒng Emperor in the Tŭmù Crisis and leading the Míng to abandon the portions of this location lying north of the Ordos Desert. The first major project establishing this location was launched in reaction to the Xiōngnú by Qín Shǐ Huáng. For 10 points, identify this location defined by a massive network of jumbled fortifications in Northern China, which is not actually visible from space.
A demilitarized zone extending south from this location was established by the Tánggū Truce.
the Great Wall of China [prompt liberally on "China," "northern China," "Manchuria," etc.; accept Dàtóng or Xuānfǔ at any point; accept Shānhǎi Pass or Tǔmù Fortress before mentioned; accept China's northern border or equivalents]