bulwark vs crenellation

bulwark and crenellation both are nouns.

Word	Noun	Adjective	Verb	Adverb
bulwark	Yes	No	Yes	No
crenellation	Yes	No	No	No

As nouns, crenellation is a hyponym of bulwark; that is, crenellation is a word with a more specific, narrower meaning than bulwark:

bulwark: an embankment built around a space for defensive purposes
crenellation: a rampart built around the top of a castle with regular gaps for firing arrows or guns

Other hyponyms of bulwark include bailey, battlement, crenelation, earthwork, fraise, merlon.

bulwark (noun)	crenellation (noun)
a protective structure of stone or concrete; extends from shore into the water to prevent a beach from washing away	the action of constructing ramparts with gaps for firing guns or arrows
an embankment built around a space for defensive purposes	a rampart built around the top of a castle with regular gaps for firing arrows or guns
a fencelike structure around a deck

bulwark (verb)	crenellation (verb)
defend with a bulwark