Stop putting 1 line shadows next to outlines. This creates pillow shading. It's very evident in the bottom stones, around the green wood, various places.
Break up large spaces, like the cieling... place various tiles or something on top so it isn't a big blank space?
Contrastttttt.... You need to look at two shades and think "does this look like a different color, or is it pretty much the same?". If it looks sorta like the same color, then you need to pick a different shade. When you do this right, you do not to place lines to show corners, the 2 contrasting shades will create a corner themselves
I'd try to tell you to learn to AA, but there isn't much to AA in big square things. :o
You could add AA to the curvy railing, the corners of some of the shadows, various things...
Don't dither.