One-way barrier (4e Trap)
From D&D Wiki
This barrier consists of a thick shield of arcane energy, with a strange feature, you can only walk through it at one side! This can be used with, for example, a dead end, a prison, or a barrier above a pit. (creatures will still fall in, but can't get out) This trap can also be very annoying when used with artillery monsters, who shoot through the barrier, while the heroes can't even throw the projectiles back!
One-way barrierLevel 5 Trap
|Detect: Arcana DC 15 to detect the barrier, DC 20 to know its powers.
|Attack: +7 vs will (the triggering creature)
|Trigger: A creature or object leaves the square with the barrier in it.
|Hit: The target treats the squares of this trap as blocking terrain that doesn't block line of sight until it takes an short rest, falls unconscious, or the trap is disabled.
|♦Disable Arcana DC 20 when the powers of the barrier are known, DC 25 when they aren't.