Welcome to Pixelation.
Right now this is relying too much on outlines only. Of course outlines can be part of the style if used well but there isn't much variation here otherwise. The slime enemy here and every tree in your first post is an example of that.
It is very rough but this edit shows how far you can go w/ well-placed blocks of flat color:
I made some props bigger and got rid of some straight outlines, like on the side walls.
And whats a dungeon level w/o torches?
Also used some "detail" tiles (chipped or moved bricks and dirty floor tiles, etc) to give it a disused look, these can be implemented easily and give you more variation at less cost.
HTH.
By the way, don't upscale your images. We have CSS code here that lets you upscale and downscale images by left and right clicking respectively.
