I'm actually working on the projectiles passing through walls issue this weekend, now that most of my higher priority jobs are complete.
I'll be making it so projectiles collide with walls, so if your units are just behind the wall, enemy units might still target them but they'll be safe unless they are back far enough that the Archers are able to angle their shots over the walls.
Look out for this in the next patch.
Edit: Just did some preliminary tests and it's looking good, it's actually pretty cool seeing a wall peppered with arrows
