A merlon is a solid, vertical part of the wall of a fort or a battlement. The narrow opening or tiny window in a merlon is called an "embrasure slit."
Merlons are always solid, and often they are the upright, pointed sections of a parapet or fortification wall. If there are wide spaces between the merlons, they are called "crenels." You can see merlons on a variety of old forts and castles, as they were commonly used from medieval times on. The Italian root is merlone, from merlo, "battlement," possibly from the Latin mergae, "two-pronged pitchfork."