I tried to test if crates were better than walls but its not very easy to do. The yellow dot is a level 5 wall that held up throughout, everything else is level 4 except for the red dots. The zombies smashed all the crates including behind the wall so I guess walls are stronger but its not very easy to tell. The blue dots are where I found zombies inside the walls. I have no idea how they got in as even the corners had pillars in the way. The buggers even got into my gatehouse this time where the pink dot is.