Now I filled most of the blank spaces with some kind of ceiling and bats,  but I couldn't make the background rocks better so I left them the way they were.

Yeah the screen is a little bit big for the tiny hero, but the huge skull demon needs the space, and I didn't know what to draw into the blank spaces if you have any suggestions it would be great . Oh and this hanging stuff is the bridge to the gap. I thought that it will lower down during the gameplay or something. And thanks for the tip about the rocks. 

the screen seems a little unneccesarily large, theres alot of blank space, and the character is pretty tiny in it.  Im not sure what the thing is hanging above the center of the room.  Im guessing some type of chandelier or something but it looks more like a clothes hanger because of the shape.  You could also try reducing the contrast in the rocks in the background, because the black outlines around them are making them seem more like the are in the foreground.