The Dungeon is a prominent location in both No Place Like Holmes and spin-off The Mary Morstan Mysteries. It is located at the very bottom of Moriarty Manor (later renamed Chambers Manor), and is the evil equivalent of Holmes' consulting room; a place where plotting, scheming and conspiracies take place. It is illuminated solely by candle light and festooned with antiques and items of an occult nature; it has remained largely unchanged in over a century.
In The Mary Morstan Mysteries, despite it having two chairs, Moriarty does not permit Gwendolyn the use of one; a trait that Madeline Chambers would repeat with Braderick a century on. However, she does later permit her second in command, Rebecca Constance, to be allowed this honour.
The dungeon was first seen in the last few minutes of the episode "Red Rising" (S3, E1) and has since been the starting and ending point for most episodes.