What is the synonym for citadel?

Here are some synonyms for citadel, depending on the specific context you're using it in:

General:

* Fortress

* Stronghold

* Bastion

* Keep

* Fort

* Castle

Figurative:

* Bulwark

* Rampart

* Sanctum

* Haven

* Sanctuary

Specific to meaning "center of power":

* Headquarters

* Powerhouse

* Nerve center

* Command center

The best synonym for you will depend on the specific context and the nuance you want to convey.