You have a number of issues here.
The worst is probably the huuuuuge lack of contrast.
Here's a considerably boosted, dirty edit (just adjusted levels in PS: some bright things were lost):
BUT since this is supposed to be underground, the solution could be totally different: a dark atmosphere broken by short-range lamps or torches. Even so, it needs more contrast.
You have 119 colors: this is clearly indicative of a problem, either in the way you manage your palette, or maybe the accidental use of a soft brush, semi transp or some other dirty tool.
There are perspective (sprites) and shading (bridge shadow) inconsistencies.
Good texture on the wall; also on the floor except the repetition is too obvious. 2 or 3 different floor tiles might help.
At this stage I'd recommend to make a few non-tiled pieces or mockups, so that you get a better idea of what your project needs, discover potential glitches and think of fixes, before committing too much time to making tiles if you're not sure of how they will work.