Definition of "citadel"

Intrenched within the citadel of our apartment, and cheered by the comfortings of a coal fire, we passed the day in letter-writing, conversation, or gazing from the sheltered security of our windows upon the agitated sea […]

1836, Alexander Slidell Mackenzie, The American in England, page 269

Twenty-two of these — eleven per broadside — were on the main deck within a central citadel, essentially an armor-protected box in the middle of the ship. Also within the citadel were four 110-pdr. breech-loaders.

2000, Lincoln P. Paine, Warships of the World to 1900