Thanks for the feedback, if I get a chance to revisit this I will take a look at those palette selections - true about the stairs/exit - just didn't put a door in at the edge... oops... 

You're missing the most important part: stairs/portal to the next level! :]

The darkest colours in some of the sprites (particularly the dark brown and grey) blend into the background. The darkest parts of the walls also don't really read as parts of the wall. If you're able to make changes to the palette, I recommend lightening those colours slightly.