Medieval Castles - Dungeons
Arguably no other part of a castle has fascinated children as much as the dungeon. Originally, dungeons formed part of the castle keep and were basically the last defensive part of the castle. However as castles became more luxurious, the dungeons became associated with prison cells and often they were located underground. Technically speaking when people talk about a dungeon what they often mean is an Oubliette. This comes from the French for "to forget". Basically a prisoner was put in here, forgotten about and left to die.
The most common form of Oubliette was a room only accessible from a hatch in a high ceiling, basically you couldn't escape without outside help.
An alternative design was a long sloping tunnel with one shelf (for the prisoner) and nothing but water or the sea at the bottom. The victim could stay on the shelf and starve or throw themselves down and drown.