fastness vs redoubt

fastness and redoubt both are nouns.

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

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

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

fastness (noun)	redoubt (noun)
a strongly fortified defensive structure	an entrenched stronghold or refuge
a rate (usually rapid) at which something happens	(military) a temporary or supplementary fortification; typically square or polygonal without flanking defenses
the quality of being fixed in place as by some firm attachment