Consider doing it with slight perspective asymmetry; at least, just the spiral. It may be too difficult to render in full top-down perspective. Otherwise, I would suggest going from darker near the bottom to lighter near the top (darkness infers depth, I think).

It may help the definition of your spiral to blur the line (which is really clear at present) between the bottom of the spiral and the floor of the cave--I imagine the surface is the same, but at the moment the spiral seems to be floating on top of the floor instead of growing out of it. Does that make sense?

The textures you used are well chosen.