Answer:
Citadel
Step-by-step explanation:
The citadel is a term that indicates the strongest fortified, usually the central part of a fort, a stronghold. A space located inside the city, yet separated. The term was derived from the Italian citadel in the early modern age, meaning "small town".
The citadel was often located inside or on the outskirts of the city and most often formed the last core of defense in a single fortress. Citadels from the early modern century, as a rule, had a regular polygon in the floor plan.