Thank you guys for your nice comments!
@Von kurst: Yes, that could have been an idea - perhaps I'll create a hatch or something, to symbolize that it was possible "earlier" to get to a lower storage area.
About the roof, I was actually building a roof top at one of the sides, but I abandoned the idea - as you say, it is an accessable piece, and won't make it more clustered. I have a tendency to create to clustered terrain, so it'll be harder to game on.
To support realism, a roof would have been a great idea - practical I think it's a bit hard.