A fort or castle, in past times, that was used to protect the people of a city if the city was attacked, or a fortress that commands a city.
1) We visited a medieval citadel in Italy.
2) The city is still commanded by a massive-stone citadel.
Middle French citadelle, from Old Italian citadella, from Middle Latin civitas which means 'city'.
castle, fort, fortess, bastion, stronghold
limit, fragility, cabin, boundary