city line vs boundary

city line and boundary both are nouns.

Word	Noun	Adjective	Verb	Adverb
city line	Yes	No	No	No
boundary	Yes	No	No	No

As nouns, boundary is a hypernym of city line; that is, boundary is a word with a broader meaning than city line:

Other hypernyms of city line include bound, bounds.

city line (noun)	boundary (noun)
the boundary of a city	the greatest possible degree of something
	the line or plane indicating the limit or extent of something
	a line determining the limits of an area