Merlon - the solid stone work between the vacant places called the Crenel.
Crenel - is the vacant place between two Merlons.
Parapet - any wall, or mound of soil/stone used for protection
When a parapet has alternating Merlons and Crenels it is known as a battlement.
The Bailey - usually refers to the inner courtyard of the Castle but sometimes it is a reference to the outer wall that surrounds the castle.
Photo left - the bailey at Arundel Castle
Moat - a wide and deep ditch that encircles a castle, it may or may not be filled with water.