Question

what is Citadel............​

Answer 1

A citadel is the core fortified area of a town or city. It may be a castle, fortress, or fortified center. The term is a diminutive of "city" and thus means "little city", so called because it is a smaller part of the city of which it is the defensive core.

Answer 2

1) A fortress, typically one on high ground above a city.

It is actually a French-originated word.

Hope it helps :)