merlon n. :the solid part of a battlement or parapet, between two openings, or crenels

crenellation
n : a rampart built around the top of a castle with regular gaps for firing arrows or guns

machicolation
n : a projecting gallery at the top of a castle wall, supported by a row of corbeled arches and having openings in the floor through which stones and boiling liquids could be dropped on attackers.

Basic principles don't change too much. It all just escalates.