Not bad, but the mid-part looks somewhat unnatural - underground isn't normally nearly that 'broken up', and it looks like clumps of grass on top of a bunch of rocks glued together (you can even see the background through the tiny gaps). I'd suggest trying to make the rocks look more blended together, or try dirt tiles. Having some kind of dirt in there on the outer portions under the grass would make it look much more natural.

Also, there are noticable seams, you should probably fix that.

Looks pretty nice actually, but like Candescence said, it looks unnatural for the grass to be on rocks like that. It would probably look better if you drew some dirt tiles for the grass, and saved the rocks for a mountain tileset. It also seems like the grass is in a different style than the rocks, but that might just be me. Some of the rocky tiles don't seem to tile quite right (rocks cut off instead of ending properly, etc.), you should probably look into that as well.

I have incorporated tiles into my project (you are mentioned in credits) - it is very early stage :). Cheers.

https://picup.it/p/5614/

You should make tileset, it will be more easier to use your textures