The answer to this question depends on which portion of the Great Wall is being referred to. The Great Wall of China is actually a series of walls built over many dynasties that spanned thousands of years.
After uniting the warring kingdoms in China in 221 BC, Emperor Qin Shi Huang decreed that the existing sections of several walls that were built from the 8th century to the 3rd century BC by these warring kingdoms be extended and expanded to protect against the northern Xiangnu empire located in present-day Mongolia. This structure, which became the first contiguous portion of the Great Wall, was finished in nine years during the reign of Emperor Qin Shi Huang from 221 to 206 BC. Further west, another portion parallel to the first wall was built during the Han Dynasty from 206 BC to 220 AC. Succeeding dynasties built the other portions to fend off the Mongols in 1368 and Manchu invaders in 1644.
The Great Wall of China stretches for 6,400 kilometers from as far north as modern day Russia to as far east to what is now Pyongyang in North Korea up to the Xinjiang province in modern day China. The Chinese name of this magnificent structure, which can be translated to “Infinitely Long Wall,” has also been often referred to as “The Long Graveyard” because of the number of people who perished building it. Myth has it that as many as several millions of people are buried underneath the walls. While interesting, this myth was later proven false by subsequent research.
Archers were the primary defense of the Great Wall of China. But because of its sheer size, it was physically and economically impossible to keep the Great Wall of China perpetually manned, so sentries were placed along strategic portions of the wall to warn against impending invaders. In case of attack, watchtowers located at intervals provided safe shelter for sentries and some soldiers while battlements with wide slits on the inside and narrow slits on the outside allow archers a wide angle shot.
To be sure, this huge structure could not repel a determined enemy and prevent him from marching into China as history has shown that invaders have broken through vulnerable portions of the wall such as the gates. Despite its weakneses, the Great Wall has at least served its purpose by slowing down the invaders and provided ample warning that bought the Chinese Empire some precious time to prepare for war.