fastness vs blockhouse

fastness and blockhouse both are nouns.

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

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

fastness: a strongly fortified defensive structure
blockhouse: a stronghold that is reinforced for protection from enemy fire; with apertures for defensive fire

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

fastness (noun)	blockhouse (noun)
a strongly fortified defensive structure	a stronghold that is reinforced for protection from enemy fire; with apertures for defensive fire
a rate (usually rapid) at which something happens
the quality of being fixed in place as by some firm attachment