down town vs metropolis

down town and metropolis both are nouns.

Word	Noun	Adjective	Verb	Adverb
down town	Yes	No	No	No
metropolis	Yes	No	No	No

down town (noun)	metropolis (noun)
the center of a city	people living in a large densely populated municipality
	a large and densely populated urban area; may include several independent administrative districts