defense vs fortress

defense and fortress both are nouns.

Word	Noun	Adjective	Verb	Adverb
defense	Yes	No	No	No
fortress	Yes	No	No	No

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

defense (noun)	fortress (noun)
(military) military action or resources protecting a country against potential enemies	a fortified defensive structure
the act of defending someone or something against attack or injury
a defendant's answer or plea denying the truth of the charges against him
protection from harm
a structure used to defend against attack
the justification for some act or belief
the speech act of answering an attack on your assertions
the defendant and his legal advisors collectively
(sports) the team that is trying to prevent the other team from scoring
an organization of defenders that provides resistance against attack
(psychiatry) an unconscious process that tries to reduce the anxiety associated with instinctive desires