Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
The entryway is lined with suits of armor.
Any opponent in this dungeon which is unarmored will gain the benefit of chainmail whenever they are struck in melee combat. When they are not being attacked, the armor goes away, removing any penalties that such armor may normally impose. This armor is made of force and cannot be bypassed by incorporeal attacks.