Hi, not discouraging other responses (or even links to other posts- there are many on the Great Wall!), but fyi, there have been a few interesting threads on the purpose/effectiveness of the wall, so you can get started on these:

• How effective was the Great Wall of China after its completion? featuring /u/bigbluepanda

• Was China's Great Wall militarily effective, and if so, for how long? featuring /u/proteinseverywhere and /u/lukeweiss

• What was the use of building the Great Wall of China in such rugged terrain? featuring /u/AsiaExpert

• Did the Great Wall of China work?

• Did the Great Wall pose any sort of challenge to Genghis Khan's invasion force?

• How effective was the great wall of China at repelling invaders? featuring /u/_dk