FYI re: Battlement (please look in your dictionary; you'll likely find illustrated a watchtower):
A parapet with open spaces that surmounts a wall and is used for defense or decoration. WEBSTER'S NEW COLLEGIATE DICTIONARY, 1977
merlon: one of the solid intervals between crenels of a battlement parapet.
crenel: one of the embrasures [an opening with sides flaring outward in a wall or parapet of a fortification usually for allowing the firing of cannon.] alternating with merlons in a battlement.
Next time a volley of arrows comes hurtling toward you, it could have come through a crenel.
CoCo Crenelated