A security function used in medieval castles where defenders can rain arrows or stones at invaders in hopes of thinning their numbers.
Castles and Palaces are two different things, a palace is the home of a monarch or lord, meanwhile a castle is a defensive fort to house the monarch or lord. Murder Holes would be uncommon in palaces (unless a monarch was particularly paranoid) but are in almost every castle.
A slang word used to describe the opening in amphibious boats hull. It was named the murder hole in World War II by the marines who would land on Omaha Beach and experience German gunfire from the trenches killing most everyone inside the boat instintly. It was very terrible to be one of the first men in the boat. It was used and showed in the movie Saving Private Ryan.
"Clear the Murder Hole!"
"Move fast and quick, watch for those mines, and clear those murder holes!"