You can try toning down the background by making it a bit less distractive while you draw. Just Return it back the way as it was before later, makes the Job easier.
Other then that, you can try using a seperate Layer to sharpen the edges and use the light from the cave to give it slightly more shape.
you can also use a technique where the front is darker and the back turns lighter in color called sfumato from trad painting. I am not that good at explaining but if you google it you should see what I mean.
Both should help a bit in making it more concrete if that is the goal, but I think its already looking pretty good.