1. A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval european architecture.
2. the act of crenellating; adding a top row that looks like the top of a medieval castle.

1. the action of constructing ramparts with gaps for firing guns or arrows
2. a rampart built around the top of a castle with regular gaps for firing arrows or guns