did this edit couple days back, never got around to posting it
I was just going to give you a different take, and explore the possibility of darker screen space and negative shapes, and also proposing the possibility of imposing some interesting loose restrictions
16Bit consoles had several tile modes that could be combined, and the ability to flip tiles, Metroid for example had 8x8 tiles that were used to great effect, and another thing the snes had was alpha transparency. So if you are working with layers, an interesting Idea is to have a lighting layer, simply solid coloured tiles with universal layer transparency.
anyway here is the edit:
and Facet has given some fantastic advice. I tried doing similar things but just by reducing highlights so the tiles didn't seem so harsh and sharp, but keeping that sharpness for the surface tiles to help distinguish them. hope it's self explanatory and in some way helpful, subtle god rays could work equally as well on your current BG if you work a 3rd subtle layer in, maybe mountains with a few openings of light between them that you could frame with pillars for a similar effect. oh yeah and I scribbled a ghost in, before you had any indication of the style of monster sprites you'd use, also one crit with your latest iteration, I don't think the spikey things without the blood read very well with that colour/shading on the prongs/spikes.