They were quite literally Murder Holes! Murder Holes were also built into Machicolations which were projecting parapets or platforms designed to hang over castle walls - similar to a balcony. These holes gained the popular name of ' Murder Holes' also called Meutrieres.
Missiles dropped from Murder Holes
The Missiles dropped from 'Murder Holes' included heavy stones, hot sand, molten lead, boiling water and boiling tar or pitch. During siege warfare any rotting materials such as dung or dead bodies would also be dropped from the murder holes!