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.)