Game is looking insteresting!
I think you should try to break off the squareness of it all. Maybe just add some slopes if you plan on doing them, otherwise just smoother transitions for the corner tiles. Ceiling tiles are especially something where you can go mad, because precise collision there is not important.
Other than that, the sky seems a bit too dark and saturated, and the gradient is pretty weird, I feel a solid colour would work better, or just using a real gradient then, or trying to break the straight lines, by shaping them into abstract clouds or something. The clouds themselves also seem too saturated, and pop a little too much. Your eyes kind of want to focus on them when they should be focusing on the characters instead.
Heres a quick edit, to illustrate some of my thoughts
