It seems wrong how the tiles fade to a flat brown based on how far away they are from the surface/cave area. It's kind of like pillow shading, no offense intended. It seems like it should be either consistent throughout or layered according to depth or varied based on where the light is. Also, because it's all right-angles, it gives away your tiling.
The character portraits look cool, but they don't seem to add anything that isn't present in the sprites themselves. I'm not sure if that's a good thing or not.