When addressing the aldermen of the City, both the Duke of Hastings Buckingham and Richard III in Shakespeare's Richard III address them collectively as "Citizens". To my knowledge this is the first such usage in the Wars of the Roses plays, and seems at odds with what (very little) I know about early modern England.
Would Shakespeare's usage have been at all common either in his own time, or in the time of Richard III? How would it have been understood by Shakespeare's audience? Are the aldermen uniquely citizens, or could this appellation been used for any English subjects?
In my understanding, "citizens' was the conventional term of address for urban gentry politicians like alderman, as the word referred to a city or town dweller, and especially to the freeholding middle and upper classes (most notions of "the people", terms for people and notions of rights in this period in Europe were skewed towards an assumed reference point of freeholding middle and upper classes, especially from a royal perspective). So while it could be used to refer to any Londoners in general, it would be most applied to aldermen, judges, sheriffs, burgesses, merchants and other more "respectable" men (and it would be mainly men) who were more active in the social circles that mattered.
It's important to note the way the royal court worked at this time. Don't think of it as institutional office, although it was also in that way like a modern executive administration, first and foremost the court was a social network. A large, mobile social network stretched across various royal residences and government offices as disconnected home-grounds. I like to think of it kind of like a beehive or a spiders web, which is emphasised by the increasingly labyrinthine structure of palaces over the 15-17th centuries. It wasn't just one discreet main office like Whitehall, which is itself composed of many rooms and individuals, it was also all the other buildings, and it never stayed entirely still. The more bureaucratic middle officials might stay in Westminster following regular work routines like a modern offices, but the monarch was on the move and many others followed. And it was also in the private houses of its important members, e.g the Theobalds and Hatfield in turn served as major places of royal activity and bureaucratic activity during the ministries of William Cecil, 1st Lord Burghley and Robert Cecil, 1st Earl of Salisbury. The Cecils, and this was during Shakespeare's time, had a whole army of secretaries, clerks and spies answering primarily to them that were almost as much a part of their household as the servants. And in this time, there was less distinction between the personal and political, domestic and bureaucratic, ceremonial and actual, compared to our modern system were officially power comes from meritocratic and/or electoral means and operates in a quite officious legalistic way. Many nobles and gentry would just show up and try to influence things solely by their implicit political role as parliament-voting elites.
So the point being, from a royal perspective the "citizens" of London they cared about were the officials and gentry freeholders who were part of that social network, which was a royal's whole world at this time. The rest of the population could be almost invisible at times, given the simultaneous size but also exclusivity of that network. Addressing aldermen as citizens was implicitly acknowledging their membership in that network as respectable burghers. Whereas the Iberian Jews, Black West Africans and other immigrants in 16th century London Shakespeare was familiar with would be much less likely to be called "citizens", nor would the aldermen's wives. The notion of "citizen" at the time had implicit associations with a political and economic status they did not quite have. It truly was a (white rich Christian) man's world and they could be pretty overt about that.