(a large strong building with thick walls and towers): This castle dates back to the 12th century.
citadel (a castle on high ground in or near a city): The quake destroyed a 2000 years old citadel in the city.
tower (a tall building or part of a castle or church): A hail of arrows descended from the nearby tower.
stronghold (a strong place that hard to attack): The castle was an important stronghold indeed in the past.
fort (a strong fortified place used by soldiers for defending an important place):
The rebels besieged the fort for two months, but still couldn’t take over it.
fortress (a large fort or group of forts, a large fortified place): It’s in fact much easier to capture a fortress from within.