Must an illusory wall be "viewed" from both directions? For example one that is over a door, clearly from one direction the door will look like a wall, but would it not also hide the door from the other side as well? Could it be created so that only one side is an illusion while the other shows reality?
Yes I can see this being an issue of being able to create fantastic sniper blinds if they can work this way.
I should mention that its not for the caster of the spell that its important, but for allies of the caster. Say, guards defending a keep. The wall doesn't look like there are murder holes, but the guards can see through them and still fire.