Look up: battlements

1. 
   Battlements
   parapet on top of a castle wall, with a series of gaps (embrasures or crenelles) between raised portions (merlons), allowing defenders to shoot through; also called crenellations
   Found on http://www.castlexplorer.co.uk/glossary.
   
   
2. 
   Battlements
   Generic term for crenallations of timber and/or stone. They were added to buildings to give cover to defenders. Crenallating, the act of building crenallations, required a licence from the Monarch without which a castle was classed as unofficial (as for Nafferton, Northumberland). Loopholes could also be used to drop heavy items onto attackers from...
   Found on http://www.keystothepast.info/durhamcc/k
   
   
3. 
   Battlements
   A form of indented parapet around the top of castles and towers which may either be defensive or decorative. A Guelf battlement was rectangular while the solid upright blocks (merlons) of a Ghibelline battlement were further indented with a 'V' shape.
   Found on http://www.arca.net/postcard/gourl.asp?U