Oubliette - Hidden hole in a floor through which people can be pushed in and kept there. Spikes are often placed at the bottom to impale the victims. It is a rare castle feature.
Ordinance Survey map (OS maps) - These are maps compiled from aerial photography and then mapped digitally. They are highly detailed and very useful. All map references on this site are based on the OS grid reference system.
Orthostat - A standing stone that helps to form the side of a chamber in a tomb.