fastness vs defensive structure

fastness and defensive structure both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastness	Yes	No	No	No
defensive structure	Yes	No	No	No

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

Other hypernyms of fastness include defence, defense.

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