(Originally) the deck of a warship on which the citadel is situated; (later also) the promenade deck or hurricane deck of a merchant ship or passenger ship.
Origin
Late 19th century.
Definition of citadel deck in US English:
citadel deck
noun
rare
(Originally) the deck of a warship on which the citadel is situated; (later also) the promenade deck or hurricane deck of a merchant ship or passenger ship.