@MysteryMeat: Contour = outline.
Try keeping the lower portions of the walls and other background elements flatter in colour, so that the characters are usually against solid colours rather than a bunch of detail. That will help them stand out even if their values aren't very different from those of the background. Currently, most of your examples concentrate the background detail in the same areas where the characters are, which hurts clarity. Your "rushed" tileset from July 26 was actually the best example in that the wall behind the characters had very low-contrast details and was mostly a solid colour, and the higher-contrast details were all high up, where the characters didn't overlap them.
It would probably also help to make it so that the sky and such are closer in value to the walls in the areas where characters overlap those elements, so that the boundaries between different objects don't form very high-contrast areas. This is currently an issue with your dark walls and light sky. Try having some darker foreground clouds, perhaps?