castle (a large strong building with thick walls and towers, built in the past to protect the people inside from attack): This castle dates back to the 12th century.
citadel (a castle on high ground in or near a city built in the past as a place where people could go for safety if their city was attacked): 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 place that is strongly built and hard to attack): The castle was an important stronghold 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 much easier to capture a fortress from within.