Dungeon entered by trapdoor

One word substitute is Oubliette.

Cellar: a room below ground level in a house, often used for storing wine or coal.
Ditch: a narrow channel dug at the side of a road or field, to hold or carry away water.
Vault: a roof in the form of an arch or a series of arches, typical of churches and other large, formal buildings.
Oubliette: a secret dungeon with access only through a trapdoor in its ceiling.