Citadel City: Oh, there have been many of these in the past. Eliéra was a friendly world, in many ways, but not that friendly, and much of history has not been as peaceful as the modern Empire. But to choose one? They speak of glacier-bound Miragrann, paranoid-built, with fortifications as much against ice and weather as invaders; of cliff-carved Stonesmight and the Granite Gates of Azikhan; of the Seat of Storms perched in the thin air of its mountain peak; the spiral walls of Cileädrin and the living-wood mazes of Verdancy; the cavern harbors of Ethring; the ancient diamond bastions of Ellenith set amid its crater labyrinth, and a dozen more…
Most, of course, buried deep within suburbs and arcologies in the modern era, but yet the defenses remain.
(The conspicuous exception being Calmiríë itself, which never had any walls or other fixed defenses until later ages made air and missile defenses necessary, despite the Empire’s enemies. That was another of Alphas I’s “little statements”.
Well, that and the capital of Ancyr, which was always unwalled for the same reason Sparta had no walls. Which was one of their little statements.)
Arcology: A prominent feature, reaching up to several miles wide and high, of the Empire’s older and higher-population worlds, where clusters of several of them in lieu of sprawling suburbs (they’re also much easier to build than suburbs on non-garden worlds, for one thing, where the distinction between “arcology” and “habitat” blurs) around a relatively conventional downtown will form the cores of cities, interspersed with lesser buildings, estates, parks, and woodland.
Urban Segregation: Averted in the Empire, at least in the prestige/class-based sense, mostly because the sort of class structure that results in people of different classes (especially wealth-based classes) declining to associate in the segregationist sense is an outgrowth of primate relative status hierarchies and exclusivist signifiers. Since the eldrae, as they would put it, are not insane – or in any case, don’t do relative status hierarchies, and people of all wealth levels share the same mores, culture, and tastes – their cities aren’t divided into the upmarket and downmarket areas, the gated and the ghetto; their districts tend to be a jumble of all incomes, and residential, commercial and light industrial properties all mixed together.
(As a side note, I would add, most of the cities are owned by people who, first, consider their personal pride a function of their civic pride; and second, as runér, are inclined to the quaint notion that poor people are every bit as entitled to have their rights respected as anyone else. Any city-district that degenerated to a level even substantially above that of a “ghetto” would swiftly attract a military garrison, loaded to shoot trouble; and even before that, well, the Watch Constabulary understand the iterated prisoner’s dilemma, and that their jobs will be a hell of a lot easier if they keep things above the point at which the rational strategy flips to default-defect.)
Inasmuch as you’re going to find differentiated districts, you’re going to find them differentiated by function, profession, or cultural fillip – “Commerce Gyre”, “City Hall”, “Cinnareville”, “Kaethburg”, “Little Vervian”, “Codewalkers’ Apartments”, “Cogging Ash”, etc., etc., with lots of mixed creole districts in between.