A citadel is a large fortress or castle that is usually built to protect cities or towns from attacks or disaster, though citadels have been built for other reasons as well. Citadels are often built to be the final protection for a city or town, in case enemies break through other protections such as walls or soldiers.
In Ancient Greece, the Acropolis was important in the life of the people, serving as a place of safety in times of trouble. It contained military supplies, food, a shrine, and often a palace or citadel. The most well-known is the Acropolis of Athens, but nearly every Greek city-state had one.
Citadels were sometimes built so strongly that enemies could take over the rest of the city, but fail to take over the city's citadel. During the Maccabean Revolt, the Maccabean rebels managed to take over all of Jerusalem except its citadel. It wasn't until 20 years later that they finally managed to take over the citadel too.
Citadels were not always built to keep away a city's enemies. During the Anglo-Dutch Wars, King Charles II of England built a citadel at Plymouth that could not only keep out enemies, but also the city's people in case they tried to rebel against the King.
Since the middle of the 20th century, citadels usually protect military center, rather than cities or towns. These citadels are built to protect the center from heavy attacks, such as aerial or nuclear bombardment. The military citadels under London are an example of this.