The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster] Crenelle

WordNet (r) 3.0 (2006):

crenellation
    n 1: a rampart built around the top of a castle with regular
         gaps for firing arrows or guns [syn: battlement,
         crenelation, crenellation]
    2: the action of constructing ramparts with gaps for firing guns
       or arrows [syn: crenelation, crenellation]