What were crenels and merlons used for in a castle?

Crenels and merlons are the parts of the battlement or parapet at the top of the walls of a castle. A parapet is a wall-like barrier at the edge of a roof, terrace, balcony or other structure. Where extending above a roof, it may simply be the portion of an exterior wall that continues above the line of the roof surface, or may be a continuation of a vertical feature beneath the roof. It becomes a battlement when portions are cut out to allow the discharge of arrows or other projectiles. The cut out portions are called crenels. The solid portions between the crenels are called merlons. When not shooting at an enemy, a soldier can take cover behind the merlons. When shooting, the soldier is at least protected by the part of the wall below the crenel so that only the upper part of the body is exposed during the shot. The merlons also protected the soldiers from arrows, etc. that might be launched from most angles other than straight-on.