sentinel vs watcher

sentinel and watcher both are nouns.

Word	Noun	Adjective	Verb	Adverb
sentinel	Yes	No	No	No
watcher	Yes	No	No	No

As nouns, watcher is a hypernym of sentinel; that is, watcher is a word with a broader meaning than sentinel:

Other hypernyms of sentinel include security guard, watchman.

sentinel (noun)	watcher (noun)
a person employed to keep watch for some anticipated event	a guard who keeps watch
	a close observer; someone who looks at something (such as an exhibition of some kind)
	a person who keeps a devotional vigil by a sick bed or by a dead body