for 64x64 your ground tile had very visible grid. I hope I helped that aspect a bit. You're right to not have a lot of lightness variation between your ground tile colors, because they're supposed to be background... but do you really need purple shadows pink highlights as if it's flesh? The project is looking great, generally.

Thank you so much for the feedback and the edit, Helm! very much appreciated! I kinda have a habbit of "purplefying" everything, so I used your colors as a base for the new tiles. I chose to scrap the old ones since I came to the conclusion that I'd better off, adding larger cracks and structures as additional tiles.
someone at PJ mentioned that the gooey flesh thing was out of perspective, so I edited that too.

Is the golden thing a spaceship? I think you overdithered a bit there; it looks a bit to busy in places. Also some more contrast in the dark areas would help to make everything a little bit more solid. you could use your darkest shade in some of the "ring areas" to give it more depth. I do line the overall color-ramp though.

keep it up!

some more progress...

the dirt stuff is pretty ugly atm; needs a lot of work. I'm pretty happy with the grass tiles though, just might need some more edge tiles I guess...
the gooey flesh thing will probably be animated, and I'll need lots of these, since I don't want to much repeating stuff...

this looks pretty awesome already! I'd agree on the readability of the claw-thingies... also, for the foots, I think a "V" -form stance is commonly used in Mech drawings, and would suit your robot best, IMO.(like in this gundam artwork http://www.mahq.net/MECHA/gundam/zz/msz-010.jpg ) also, something seems of about the wind-key at it's back. There's a highlight on (lower)right, although the form suggests that it shouldn't be there...

still pretty impressive!

EDIT: dang... to late with my reply...