Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
This large round holding cell has several rings and hooks and the walls and ceilings, presumably to chain prisoners to.
A huge spring is under the floor. At some point, a loud clicking noise is heard as the spring uncoils, propelling the floor into the ceiling.
Those next to the walls take average slashing damage from the hooks and rings, subject to a hard dexterity saving throw. Everyone takes heavy damage when the floor smashes into the ceiling. Targets within half their move speed from the doorway may make a hard dexterity saving throw to quickly exit the room.