blockhouse vs fastness

blockhouse and fastness both are nouns.

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

As nouns, fastness is a hypernym of blockhouse; that is, fastness is a word with a broader meaning than blockhouse:

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

Other hypernyms of blockhouse include stronghold.

blockhouse (noun)	fastness (noun)
a stronghold that is reinforced for protection from enemy fire; with apertures for defensive fire	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