Most models' heads should be able to see over the defense line or through the armor slits. The line is meant to give them cover, blocking well over 25% of them but not the heads. Line of Sight is from the eyes, so the wall should not interfere. And while I don't have my book handy, I do believe models in base contact ignore walls when firing over them. It's sort of like being within 2 inches of the edge of a forest and firing out. You get cover but not the enemy.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."