@unity: It's a great first go at pixel art, but there are a few tips I'd give you going forward.

1) Remember to always have a light source in mind when shading. It's usually to the top left or right, but it can be anywhere, so long as it stays static. The end result should be that shades closer to this fictional light source brighter and the ones further away are darker. This allows you to show curves (in cliff walls and such) very easily, and adds a great deal of depth. This also helps to avoid pillow shading (where all sides of an object are shaded in the same way light-to-dark, making it look rounded and pillowy).

2) Either don't use black or use lots of dark edging shades. There is black in your floor tile, but then you cliff tiles use a very light border colour. You don't have to use black in the cliffs (I wouldn't recommend it, even) but you can use a very dark brown without it breaking the style.

3) User higher contrast palettes. Going light-slightly less light-medium-slighty darker-sort of dark is fine, but experiment with more pronounced palettes (go from very dark to very light) and add in tones of other colours (look at a colour wheel to determine which hints go where to create high-readability/high-contrast floor/wall combinations). The more you experiment, the more you'll find new colours you real like. Judging from your other screens (and this one) I don't think it'll be much of a worry.

4) bonus tip: Avoid tile-patterning in floor tiles. The floor tiles used here forms a very visible repeating line, which you want to avoid. Play around with less distinct patterns in the floor, or use the venerable four-tile floor, which usually helps to break it up. I notice the pattern is broken in one or two spots, so you must have another dirt tile in there somewhere, and that's a good start. The best tiles don't look like tiles.

I don't mean to rag on you, I hope these tips help!