Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
A long drawbridge leads to a gatehouse.
When the drawbridge is crossed, it causes the gates on the gatehouse to shut, and the drawbridge snaps closed like a mousetrap. All creatures on the drawbridge may make an average Dexterity saving throw to be able to jump off of the drawbridge. They will take 4d6 falling damage.
Otherwise, those on the drawbridge are hurled into the gatehouse, taking 4d6 bludgeoning damage. These creatures then may try to move out of the way with an average Dexterity saving throw with disadvantage. Those that succeed tumble away to the side, while those that fail take 6d6 bludgeoning damage from the drawbridge, and are restrained.
Restrained creatures must make a hard Strength check in order to free themselves, or the drawbridge needs to be lowered from the inside.