I am not a good spriteartist, so no crit for the characters today. But apart from that, i give you some advice for the background:
Your BG looks flat. Even though Gil made it better by making the floor darker and giving more perspective, it still looks odd.
Usually the reason for this lies behind the colours. Remember, colours are the key element of the atmosphere, depth and mood. And depth is connected to perspective. So, adding the right colours you also apply the illusion of more realistic perspective.
My advice from now is that make theinner cornes casting more shadows, that means that make the lower part of walls darker etc, this really creates nice effects. But don't use the same blue on shadows, pick another tone, something warmer. You should know that cold colours usually have warm shadows and vice versa. In your piece, I suggest some brown for shadows and some lighter orange with the mix of light blue for highlights. So, basically, use more depth - closed areas (eg inner corners, etc) have less light bounciong back to the viewer, they swallow.