Thank you both for your kind words and helpful advice.
Unfortunately CC3 doesn’t have layer masks, or easy methods to apply gradients to selective areas of the map. However I could set up the rubble in GIMP and then import it back into the CC3 map. In which case I’d be able to take on board everything that both of you have suggested. Thanks!

@madcowchef: Hee hee, the “background” does indeed look like it’s above the floors/walls. I liked the style of these floorplan tiles, that Kristian Richards of The Crooked Staff, posted on Google+ (I can’t find any posts by him on this forum unfortunately). So I wanted to try and have a similar raised, bevelled “surface” above the walls for my dungeon map.