Oubliette

n.

1.

A dungeon with an opening only at the top, found in some old castles and other strongholds, into which persons condemned to perpetual imprisonment, or to perish secretly, were thrust, or lured to fall. Sudden in the sun An oubliette winks. Where is he? Gone. - Mrs. Browning.

Noun

1.

oubliette - a dungeon with the only entrance or exit being a trap door in the ceiling

dungeon