"fortress commanding a city," 1580s, from French citadelle (15c.), from Italian cittadella, diminutive of Old Italian cittade "city" (Modern Italian citta), from Latin civitatem (nominative civitas; also source of Portuguese citadella, Spanish ciuadela; see city).
"ancient Greek city-state," 1894, from Greek polis, ptolis "citadel, fort, city, one's city; the state, community, citizens," from PIE *tpolh- "citadel; enclosed space, often on high ground; hilltop" (source also of Sanskrit pur, puram, genitive purah "city, citadel," Lithuanian pilis "fortress").
15c. Inca citadel high in the Andes Mountains of Peru, from Quechua (Inca) machu "old man" + pikchu "peak."
"furnished with turrets and battlements," 1670s, from Medieval Latin castellatus "built like a castle," past participle of castellare "to fortify as a castle, build as a castle, furnish with turrets and battlements," from Latin castellum "castle, fort, citadel, stronghold" (see castle (n.)). Related: Castellation.
"a safeguard," c. 1600, originally (late 14c., Palladioun) "a sacred image of Pallas Athene," from Old French palladion, from Latin palladium, from Greek Palladion, noun use of neuter of Palladios "of Pallas." It stood in the citadel of Troy and the safety of the city was believed to depend on it. Related: Palladian.