An apodyterium was a room in Caesar Nero's palace. The First Doctor and Vicki entered the room after Tavius suggested they visit — only to discover there the body of the centurion who had escorted them to the palace. (DW: "All Roads Lead to Rome")
Nero's apodyterium
The Romans never properly defines what an apodyterium. In the real world, it was a room for disrobing.
