Seven Hills of Rome vs Eternal City

Seven Hills of Rome and Eternal City both are nouns.

Word	Noun	Adjective	Verb	Adverb
Seven Hills of Rome	Yes	No	No	No
Eternal City	Yes	No	No	No

Seven Hills of Rome (noun)	Eternal City (noun)
the hills on which the ancient city of Rome was built	capital and largest city of Italy; on the Tiber; seat of the Roman Catholic Church; formerly the capital of the Roman Republic and the Roman Empire