Invisible Wall

I put an invisible wall in this map using a func_illusionary entity and giving one side {invisible texture (I think I could've used {blue also) and gave the other side a cement texture. Even though this invisible wall is quite obvious, if you fiddled with the lighting and put it in a good place, I think it could be pretty cool in mp maps. Remember the good 'ol days with Doom? As I recal, there were a couple of invisible wall in that game too. I've never seen this done before, but I could be wrong. I'm surprised I haven't.