a rampart built around the top of a castle with regular gaps for firing arrows or guns
<noun.artifact>
the action of constructing ramparts with gaps for firing guns or arrows
<noun.act>
Crenelation \Cren`el*a"tion\ (-?"sh?n), n. The act of crenelating, or the state of being crenelated; an indentation or an embrasure. [Written also {crenellation}.]