metropolis vs Rostov on Don

metropolis and Rostov on Don both are nouns.

Word	Noun	Adjective	Verb	Adverb
metropolis	Yes	No	No	No
Rostov on Don	Yes	No	No	No

metropolis (noun)	Rostov on Don (noun)
people living in a large densely populated municipality	a seaport on the Don River near the Sea of Azov in the European part of Russia
a large and densely populated urban area; may include several independent administrative districts