Basic Map and Light Improvements

[OriginalUsername]

I'm not an expert, not even close, but what helped me a lot is just using common sense. Does this room have a purpose? Why would they need another corridor here? Where did this giant rock come from? Why did this sound come out of this wall? 
Just those kind of questions. Including particle systems, of course. If you're in a sewer, add some fog/dust/flies/etc. Make the walls dirty, add some loose rocks, just make it like a sewer.
Same goes for a mansion. Why would they need a table here? Does it have a purpose? 
For lighting: Just look around in your room. Turn off the lights, close your curtains and look around how the light still spreads in your room. Do you see those really strong and bright light rays coming through the curtains? Unless you're making a story in some kind of alternate universe, everything should match the real world. (With some creative alterations, of course)
Just think about that, and good luck