Citadel A citadel is a fortress protecting a town , sometimes incorporating a castle . The term derives from the same Latin root as the word " city " , " civis " , meaning citizen . In a fortification with bastion s , the citadel is the strongest part of the system , sometimes well inside the outer walls and bastions , but often forming part of the outer wall for the sake of economy . It is positioned to be the last line of defence should the enemy breach the other components of the fortification system . A citadel is also a term of the third part of a medieval castle , with higher walls than the rest . It was to be the last line of defence before the keep itself . In various countries , the citadels gained a specific name such as " Kremlin " in Russia or " Alcazaba " in the Iberian Peninsula . In European cities , the term " Citadel " and " City Castle " are often used interchangeably . The term " tower " is also used in some cases such as the Tower of London and Jerusalem ' s Tower of David . However , the Haitian citadel , which is the largest citadel in the Western Hemisphere , is called Citadelle Laferrière or simply the ' Citadel ' in English . History . 8000 BCE – 600 CE . In Ancient Greece , the Acropolis ( literally : " peak of the city " ) , placed on a commanding eminence , was important in the life of the people , serving as a refuge and stronghold in peril and containing military and food supplies , the shrine of the god and a royal palace . The most well - known is the Acropolis of Athens , but nearly every Greek city - state had one - the Acrocorinth famed as a particularly strong fortress . In a much later period , when Greece was ruled by the Latin Empire , the same strongpoints were used by the new feudal rulers for much the same purpose . 167 – 160 BCE . Rebels who took power in the city but with the citadel still held by the former rulers could by no means regard their tenure of power as secure . One such incident played an important part in the history of the Maccabean Revolt against the Seleucid Empire . The Hellenistic garrison of Jerusalem and local supporters of the Seleucids held out for many years in the Acra citadel , making Maccabean rule in the rest of Jerusalem precarious . When finally gaining possession of the place , the Maccabeans pointedly destroyed and razed the Acra , though they constructed another citadel for their own use in a different part of Jerusalem . 3300 – 1300 BCE . Some of the oldest known structures which have served as citadels were built by the Indus Valley Civilization , where the citadel represented a centralised authority . The main citadel in Indus Valley was almost 12 meters tall . The purpose of these structures , however , remains debated . Though the structures found in the ruins of Mohenjo - daro were walled , it is far from clear that these structures were defensive against enemy attacks . Rather , they may have been built to divert flood waters . 500 – 1500 CE . At various periods , and particularly during the Middle Ages , the citadel - having its own fortifications , independent of the city walls - was the last defence of a besieged army , often held after the town had been conquered . A city where the citadel held out against an invading army was not considered conquered . For example , in the 1543 Siege of Nice the Ottoman forces led by Barbarossa conquered and pillaged the town itself and took many captives - but the city castle held out , due to which the townspeople were accounted the victors . 1600 – 2000 CE . As late as the 19th century , a similar situation developed at Antwerp , where a Dutch garrison under General David Hendrik Chassé held out in the city ' s citadel between 1830 and 1832 , while the city itself had already become part of the independent Belgium . In time of war the citadel in many cases afforded retreat to the people living in the areas around the town . However , Citadels were often used also to protect a garrison or political power from the inhabitants of the town where it was located , being designed to ensure loyalty from the town that they defended . For example Barcelona had a great citadel built in 1714 to intimidate the Catalans against repeating their mid - 17th - and early - 18th - century rebellions against the Spanish central government . In the 19th century , when the political climate had liberalised enough to permit it , the people of Barcelona had the citadel torn down , and replaced it with the city ' s main central park , the Parc de la Ciutadella . A similar example is the Citadella in Budapest , Hungary . The attack on the Bastille in the French Revolution - though afterwards remembered mainly for the release of the handful of prisoners incarcerated there - was to considerable degree motivated by the structure being a Royal citadel in the midst of revolutionary Paris . Similarly , after Garibaldi ' s overthrow of Bourbon rule in Palermo , during the 1860 Unification of Italy , Palermo ' s Castellamare Citadel - symbol of the hated and oppressive former rule - was ceremoniously demolished . The Siege of the Alcázar in the Spanish Civil War , in which the Nationalists held out against a much larger Republican force for two months until relieved , shows that in some cases a citadel can be effective even in modern warfare ; a similar case is the Battle of Huế , where an NVA division held the citadel of Huế for 26 days against roughly their own numbers of much better - equipped US and South Vietnamese troops . 2000 – Present CE . The Citadelle of Quebec ( 1673 – Present ) still survives as the largest citadel still in official military operation in North America after more than two hundred years of existence . Naval term . On armored warships , the heavily armored section of the ship that protects the ammunition and machinery spaces is called the citadel . The safe room on a ship is also called a citadel .