After I rendered it out, the lighting is completely wrong. Nick told us that there should be one light coming from the left corner, and the other light is coming from the right corner which hides behind the wall. The left light should be pointing down. And we should look at the shadow of the stones on the ground as out lighting reference.

The shadow is very solid and has hard edges. So I created a spot light on the right side.

I used CTRL+T to change the target of the spotlight. And I put a cube as a reference to see how the light works on the shadow. I also turned down the shadow density to modify the darkness of the shadow.