Forbidden City vs national capital

Forbidden City and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
Forbidden City	Yes	No	No	No
national capital	Yes	No	No	No

Forbidden City (noun)	national capital (noun)
a walled section of Beijing that encloses the palace that was formerly the residence of the emperor of China	the capital city of a nation
the sacred city of Lamaism; known as the Forbidden City for its former inaccessibility and hostility to strangers