Oh, and I've decided to drop the white line. I hope it's not the wrong move though 
I tried moving around your character on your mockup without white outline, and it seems readable everywhere (including on very black areas). You might want to lower contrast on the pilar, though (right now, i'd be unsure whether it is foreground or background object.
Gems can easily get rid of the white outline as well if you animate them to make them "blink to white" from time to time.
depth in your brickswall : i love them
skeleton skull : it looks oddly horizontal, and should rather "fall backwards". It might have larger "eyeholes", though it is cute this way.
the vines : they still require some work, imho, maybe an alternate palette.
the spikes: they are hard to read over the pilar, surprising for such a dangerous thing.
But i love the overalllook.