The use of Post here instead of Send is a little tricky.  I assume you're doing it so that you don't modify the list as you're iterating over it, which makes sense.  The one thing I'm not totally clear on is how the delayed delete prevents the wall element from doing damage to the player as he enters the room.  Do you know?  I'm assuming that you've tested it and it works, but I'm not sure why it works.