defense vs fastness

defense and fastness both are nouns.

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

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

defense (noun)	fastness (noun)
(military) military action or resources protecting a country against potential enemies	a strongly fortified defensive structure
the act of defending someone or something against attack or injury	a rate (usually rapid) at which something happens
a defendant's answer or plea denying the truth of the charges against him	the quality of being fixed in place as by some firm attachment
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