stronghold vs redoubt

stronghold and redoubt both are nouns.

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

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

Other hyponyms of stronghold include bastion, citadel, blockhouse, hold, donjon, dungeon, keep.

stronghold (noun)	redoubt (noun)
a strongly fortified defensive structure	an entrenched stronghold or refuge
	(military) a temporary or supplementary fortification; typically square or polygonal without flanking defenses