The walls are made of stones and bricks in mountainous areas while they are made of sand and local plants in desert, you know, it is depended on the local geography. And the wall was built to defend the attacks from the northern nomadic tribes.

Bricks. Also in those bricks are bones. True story

The wall was made of a lot of things like sand, bricks, and even dead peoples bodyparts. I am positive that this is true.