Added a new level, overgrown temple type thing:
I'm pretty stuck as to what to have in the background, the standard bricks are boring me.
I definitely love the look of your mockups ^_^ but it seems to me that you haven't chosen whether that overground temple is open or closed area (if open, why such a brick wall -- if close, what do plants live from ?). If it's something like a formerly-closed-area-which-ceiling-damaged, we might have nice lightning effects, etc.
I guess since it's a temple, you might love to have statues and large doors instead of plain brick walls ... Since it's overgrown, you might very well have huge vines cracking the stones of the main hall or something ... Bg. of "Fury of the Furries" did a nice job in introducing such subtle (and humoristic) features to break monotony.
Another thing you might want to ask is "this is the temple of what, anyway?" That could give a direction to what kind of statues you'll have, include replicas of the "deity"'s face on large rocks or whatever.
Anyway, go on. This is going to be outstanding. It makes me feel the urge to complete my NDS-2D gaming engine and import your tiles to see it moving ^_^