urban center vs Rostov on Don

urban center and Rostov on Don both are nouns.

Word	Noun	Adjective	Verb	Adverb
urban center	Yes	No	No	No
Rostov on Don	Yes	No	No	No

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