作为宏大的军事工程，长城虽然既有通风报信的烽火台，又有城垛和瓮城之类用作攻防的工事，但建成后，长城的军事功能在战役中并没有发挥多大作用。更多的时候，长城以其强大的威慑力量，起到了吓阻外敌侵扰，避免战争的作用。在冷兵器时代，长城的威慑作用是卓有成效的。在广阔的草原、荒漠上，长于骑射的游牧民族纵横驰骋，锐不可挡。长城的出现，使他们不得不勒住战马的缰绳，想方设法攻克长城。但攻坚步战不是他们擅长和习惯的作战方式，于是，长城这条横亘在骑兵面前的高大墙体，消解了他们的锐气，延缓甚至完全阻止了他们的铁蹄挥师南下。
秦始皇筑长城，“却匈奴七百余里，胡人不敢南下而牧马。”
汉武帝(前141-前87年在位)，“建塞脚，起亭燧，筑外城，设屯戍以守之，然后边境得用少安。”
明成化七年(1473年)，延绥巡抚余子俊大规模修筑边墙，“北虏知不能犯，遂不复入套”。
明隆庆五年(1571年)，由于边墙加固，戒备加强，迫使俺答汗和明王朝签订《隆庆和议》。
戚继光(1528-1588年)任蓟镇总兵后，修边墙，筑敌台，积极训练兵卒，蒙古朵颜等部曾打算入侵中原，但慑于戚继光防御威严而未敢行动，“蓟门宴然”，数十年没有战事。
A Defense Work to Avoid Wars
As a massive military presence, the Great Wall was complete with watchtowers for relaying information and battlements and protective citadels for defensive purposes. The truth is that the Great Wall never had much opportunity to prove its military role in battles. Most of the time, however, it did serve the purpose of avoiding battles by scaring off any potential invaders.
In the years before firepower, the Great Wall had an obvious effect as a deterrent. It was impossible to hold in check nomadic peoples living on vast expanses of grassland and desert who were excellent at riding horses and fighting on horseback. However, for the nomadic tribes, fighting at close range to take a fortified defensive work was not something with which they were most comfortable. So the Great Wall forced them to pull back the reins of their horses and to try to come up with inventive ways to overcome it. Consequently, the mighty structure dampened their morale, slowed them down and even totally held them in check as they attempted to march southward.
The Great Wall built by the First Emperor of Qin “held off the Xiongnus from the territory for more than 300 kilometers, making them afraid to make any southward moves.”
Emperor Wu of the Han (r. 140-87 BC) had fortresses and watchtowers erected and an outer wall built. He dispatched troops to guard the wall and ensured stability along the border.
In 1473, Yu Zijun, a defending general in the Yansui region, engaged in massive construction on the border wall so that northerners dared not move near the area.
The year of 1571 saw the conclusion of the Longqing Agreement between the Tartar leader and the Ming court thanks to the strong and vigilant defense along the wall.
When General Qi Jiguang was made commander-in-chief of the garrison at Ji, he had walls and watchtowers built and drilled his troops, both cavalrymen and infantry soldiers. As a result of General Qi’s strong defensive display, the northern tribal Mongolians who had planned to invade the Central Plain never dared to put their plan into action. For dozens of years, the border enjoyed peace and stability.
