Embrasures. " An " embrasure " is an opening made in the parapet for a gun to fire through. The names of its parts are the " sole," the " sill," the "cheeks," the "Deck," and the "mouth." Embrasures should be at least 15 feet apart, in order that the " merlon " or parapet between them should not be too weak. Mantlets or shutters of wood or iron, fascines, "c., are sometimes made to close the neck, (which is only 2 feet wide), as a protection from musketry fire. For field guns the sill should be 3"

http://www.forgottenbooks.com/readbook_text/Guide_to_Promoti...

https://books.google.pt/books?id=0tSdRTsP4QEC&pg=PA213&lpg=P...

http://www.omnilexica.com/?q=embrasure