Стан-Штадт-Сити,отсюда и Ситадель.

Крепость,находящаяся в Сити.

citadel
1586, "fortress commanding a city," from It. cittadella, dim. of cittade "city," from L. civitatem (see city).

И английское "ситизен"-гражданин,житель Сити...тоже оттуда пошло:

citizen
c.1314, from Anglo-Fr. citezein (spelling alt. by infl. of denizen), from O.Fr. citeain, from cite (see city), replacing O.E. burhsittend and ceasterware. Sense of "inhabitant of a country" is late 14c. Related: Citizenry (1819).

http://www.etymonline.com/index.php?l=c&p=26