Over Grown Mossy Stone Tower Ruins Work in Progress Please Lend me your Brain Juice!

i am getting to the point where i feel like adding more will either make it or break it, i am leaning towards implementing some wood to the towers but i would love some feedback on my direction and where you think i should go! this pieces are meant to be and are modular

they are hollow and you can go inside them and i plan on adding some interior props to go along with them

i am having trouble lighting up the insides of them with my renders though, in blender i just cannot do it for some reason i put lights all around them and they still show up black on the insides any insight would be great

they are currently using 4k Stone Diffuse Normal and Displacement Rendered in Blender Cycles 2.76

and the grass is using the same material setup but with a grass texture instead