If the action moves inside I move it off table to a different indoor board, if it’s simply troops occupying a building shooting at things outside I put a marker on the building.
At this scale, exactly what I do (unless playing a more abstract game like DS2 where it doesn’t matter as much).
Those duplo do sort of work. Could be easy to glue on some plasticard and whatnot to create depth and detail quite easily. Might have to give this a try…
"I saw this in a cartoon once, but I'm pretty sure I can do it..."