Answer:
Although a useful deterrent against raids, at several points throughout its history the Great Wall failed to stop enemies, including in 1644 when the Qing troops marched through the gates of the Shanhai Pass and replaced the most ardent of the wall-building dynasties, the Ming, as rulers of China proper.
Step-by-step explanation:
The main reason for the wall's construction had always been defense. Since China's first dynasty, the empire faced a near-constant threat of attack from the nomadic tribes to the north. This included those famed Mongols as well as lesser-known groups like the Xiongnu and the Göktürks.