Hello Helkor
I don't use PS, so I can only be of limited assistance from your point of view as far as technical advice goes, but what I can do is give you a few battle map tips.
You are right - there wouldn't really be any shadows inside a house, unless you had a light source like a room light, a lamp or a fire. Other than that you are more likely to have light rays and light shadows coming in through windows and open doorways.
The other thing I could mention, if you don't mind, is that your grid is dominating the picture. It only needs to be barely there - just enough that the players can see it, otherwise it will greatly detract from the clarity and beauty of your map. Having done a couple of battle maps myself just recently I can tell you that the grids on mine are more than 50% transparent, hairline thick, and subtly softened by the use of what I call a glow (and which I think you would probably call a stroke), but only a very, very subtle one.
Hope that helped a bit, even though I don't know very much about PS