build up vs fortify

build up and fortify both are verbs.

Word	Noun	Adjective	Verb	Adverb
build up	No	No	Yes	No
fortify	No	No	Yes	No

As verbs, build up and fortify are synonyms defined as:

Other synonyms of build up include arm, gird.

build up (verb)	fortify (verb)
form or accumulate steadily	add alcohol to (beverages)
enlarge, develop, or increase by degrees or in stages	make strong or stronger
change the use of and make available or usable	add nutrients to
bolster or strengthen	prepare oneself for a military confrontation
prepare oneself for a military confrontation	enclose by or as if by a fortification