Hmm.. Since you're already using four palettes nearly to their limits, it might be hard to improve upon the background, but I think it feels a bit too bland as is...
Without changing anything to the tiles themselves, I think what you can first and foremost do is to change the set up of the background; move elements around to spice up the background. A good way to go at it is not to have everything in the same column. You might also want add some sort of conventional ceiling. That way the player's focus is drawn to where it needs to be.
When it comes to changing something to the tiles, I think the third palette group is an area of attention. That palette is used to create the wooden pillars going across the background, but those stand in very sharp contrast with the primary background color. I think it would help if you could give those pillars a more defined three dimensional shape, as well as finding a way to blend them with the primary background color more; some sort of shadow effect cast onto the wall might do the trick.
Aside from all that, though, I think you definitely got the theme of the area right. Just spice it up and it should be good to go.