There're some mojor flaws which require fixing.
1. Overstretched textures of columns etc on first two screens. It is solvable by making the same thing from smaller decorations, and gives nice detailed feel about it. You can go artistic and make it all puzzle- and pattern-like
2. Whole structure is just flat board with walls. You could make much moar with fake balconies, "second" floors etc.
3. Transparent tile you used to make it look floaty. The problem is not actually in the tile, but in the poor quality (well, you didn't have much choice anyway) sky behind it. Take some eerie fog \ simple fog decorations, add some clouds...