A citadel is the core fortified area of a town or city. It may be a fortress, castle, 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.In a fortification with bastions, the citadel is the strongest part of the system, sometimes well inside the outer walls and bastions, but often forming part of the outer wall for the sake of economy.