If you look at the top of the walls of a typical castle they have taller and shorter parts, the shorter parts were to allow archers to shoot their bows through them, the higher ones to shelter behind. It looks like a kind of saw blade. This is called crenellation.

A rampart is a protected fighting platform for castle defenders. A defensive bank of earth or rubble. Iis placed behind the crenellation.

The term embrasure, in military architecture, refers to the opening in a crenellation or battlement between the two raised solid portions or Merlons, sometimes called a crenel or crenelle.

Crenellations, also called battlements, are the defensive structures found on many castles. They consist of merlons (the higher parts which were used to hide behind) and crenels (the gaps you can use to shoot through).

Crenellation (note the spelling) is the pattern one sees on top of a wall in a "typical" castle - also known as the battlements - it is the sequence of regularly spaced squared openings to shoot through - with the raised part offering a place to withdraw behind for protection. An archer could step into the gap, fire an arrow at the enemy and step back for protection from enemy projectiles.