A citadel is a fortified structure designed to provide protection during a battle. So, back in the days when pillaging was commonplace, it was a good idea to know where your nearest citadel was.
Among the most historically famous citadels are the Acropolis in Athens and the Tower of London. Even though we're building fewer physical fortresses these days, citadel remains a useful word, particularly when you need to create a metaphor suggesting strength and safety. As poet John Keats wrote, "[I]t appears to me that almost any man may like the spider spin from his own inwards his own airy citadel."
