BOXES and pieces of cardboard attached to wall like blocks work great.
We painted boxes grey with black shading and put on the floor (the base of the wall). If you can, another suggestion is to fasten some smaller boxes to the walls to look like rocks on the wall - we have not done this but it can be done without damaging the "paint". Don't forget the "cracks" in the wall!!
BOXES can make great "village walls"
You can also hang a large sheet of butcher paper and glue squares of cardboard "bricks" to it
