stronghold vs fastness

stronghold and fastness both are nouns.

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

As nouns, stronghold and fastness are synonyms defined as:

stronghold (noun)	fastness (noun)
a strongly fortified defensive structure	a strongly fortified defensive structure
	a rate (usually rapid) at which something happens
	the quality of being fixed in place as by some firm attachment