From Italian 'cittadella,' diminutive of 'citta' (city) — literally a 'little city,' the fortified inner core of a town.
A fortress, typically on high ground, protecting or dominating a city.
From French 'citadelle' (small fortified city), from Italian 'cittadella' (small city, fortified stronghold), a diminutive of 'città' (city), from Latin 'civitas' (citizenship, community of citizens, city), derived from 'civis' (citizen), from PIE *ḱei- (to lie, settle, be at home). This root yielded Old English 'hīwan' (household members), Sanskrit 'śēte' (he lies down), and Greek 'keîmai' (to lie). The sequence from 'civis' to 'citadel' traces a political-military path
'City,' 'citizen,' 'civic,' and 'civilization' all share the same Latin root.