A strong castle in or near a city, where people can shelter from danger, especially during a war. (Cambridge Dictionary)
In the past, a citadel was a strong building in or near a city, where people could shelter for safety. (Collins Dictionary)
A fortress that commands a city. (Merriam – Webster)
A fortress, typically one on high ground above a city. (Oxford Dictionaries)
The citadel. The Military College of South Carolina.