fastness vs keep

fastness and keep both are nouns.

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

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

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

fastness (noun)	keep (noun)
a strongly fortified defensive structure	the main tower within the walls of a medieval castle or fortress
a rate (usually rapid) at which something happens	a cell in a jail or prison
the quality of being fixed in place as by some firm attachment	the financial means whereby one lives

fastness (verb)	keep (verb)
	prevent (food) from rotting
	prevent the action or expression of
	stick to correctly or closely
	maintain by writing regular records
	supply with necessities and support
	hold and prevent from leaving
	retain possession of
	look after; be the keeper of; have charge of
	retain rights to
	maintain for use and service
	have as a supply
	store or keep customarily
	stop (someone or something) from doing something or being in a certain state
	allow to remain in a place or position or maintain a property or feature
	conform one's action or practice to
	behave as expected during of holidays or rites
	cause to continue in a certain state, position, or activity
	supply with room and board
	maintain in safety from injury, harm, or danger
	fail to spoil or rot
	continue a certain state, condition, or activity
	to rear