Spurred on by Father Steve:

These two definitions come from the Online Etymology Dictionary:

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 1380s.

I wonder what brought about the sense of "inhabitant of a country"? That is not explained here. It is also interesting that the 'z' came into English from the French, not from the American!

For what it's worth, here is the entry for 'city':

city - c.1225, from O.Fr. cite, in medieval usage a cathedral town, but orig. meaning any settlement, regardless of size (distinction from town is 14c., though in Eng. it always seems to have ranked above borough), from earlier citet, from L. civitatem (nom. civitas) orig. "citizenship, community of citizens," from civis "townsman," from PIE base *kei- "to lie, homestead." The L. word for "city" was urbs, but a resident was civis. Civitas seems to have replaced urbs as Rome (the ultimate urbs) lost its prestige. City hall first recorded 1675; city slicker first recorded 1924 (see slick); both Amer.Eng. Inner city first attested 1968.