Simple:
* The ancient citadel stood atop the hill, overlooking the city.
* The rebels were trapped within the citadel.
Figurative:
* He built a citadel of silence around himself, refusing to speak to anyone.
* The government was determined to maintain its citadel of power.
Specific:
* The Citadel in Jerusalem was once a massive fortress.
* The Citadel of Quebec is a popular tourist destination.
Literary:
* The wind howled around the citadel, its walls echoing with the ghosts of the past.
* He felt like a solitary prisoner in the citadel of his own mind.
The specific meaning of "citadel" will depend on the context, but it generally refers to a strong, fortified place or a place of power or strength.