It is a huge project. It takes about 2,000 years to make the wall.

Thank you!

Why did Emperor Qin want this? Was it mainly for defence? May I know the brief history behind this?

The Emperor Qin connected the walls which once belonged to other six ducal states and built some more sections after he unified China, because at that time, the northern nomadic tribes often went south to invade the land and kill people. In order to protect themselves being invaded, the Emperor Qin ordered to build a long wall which started from Lintao county in Gansu and ended at the present eastern Liaoning Province in the east. For more, you can see this page:
https://www.travelchinaguide.com/china_great_wall/history/qin/

it took several years. each emperor tht came to power added more peices to the construction.