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.)
Shining City: Oh, my, yes. If there is a city anywhere in the Empire that isn’t competing in these leagues, it’s because it’s pursuing some other stylistic city trope just as hard as it can.
And in various styles, of course, riffing off their general tastes in Art Deco, Crystal Spires, and Raygun Gothic: the seashell spirals of Cileädrin, the black stone geometries of Leirin, the coral spires and glass-blown bubbles of Lochrannach, the living wood palaces of Veranthyr, the flying – well, hanging off the end of an orbital elevator – multi-leveled golden towers of Mer Covales, the mother-of-pearl pleasure domes of Ameri, the crystal domes of undersea Alaerlor, the vaulted, polished caves of Azikhan, the glittering gemstone pillar-arcologies of Dal Shan, the amber-and-marble mandala streets of Ellenith, the three-dimensional fractal layout of Voxelville, the mile-wide cavern parks of Silverfall City… and so forth.
While enabled by post-scarcity economics, it’s not a product of them and predates then considerably. It’s a product of a species and a culture with very firm ethical opinions on the subjects of beauty, wealth, and excellence, and equally firm aesthetic opinions on visible manifestations of entropy1. Helped along, for that matter, by certain necessities of leadership of a people who universally fail to intimidate worth a damn and are very bad at responding to crude bribery, but who can be impressed. Ergo, dear city founder, you must manifest impressiveness, and architecture is a good place to start.
The ultimate example, of course, is the Empire’s capital, “Eternal Calmiríë, the jewel at the heart of the World”, founded back in the day by Alphas I Amanyr and Seledíë III Selequelios, such that neither existing pre-Imperial capital would have priority over the other. Founded – and bear in mind this was well before the local Industrial Revolution – using the simple principle of going to the biggest damn mountain on the Cestian continent, and saying, “That? Right there? Make it a city.”
Note: not build a city on it. Not even build a city in it, although both of those things happened as part of the project. Turn the entire mountain above and below into a city, complete with all the soaring towers, shining buildings, garden parks, multi-hundred-foot-high statues, fountains, waterfalls, monuments, promenades, giant Tesla coils, shining aureoles of fey light, etc., etc., to be expected of the stone upon which the Dragon Throne rests, the temple of unsurpassed grace and shining beauty, the seat of wisdom ever-growing and power never-failing, and so on and so forth.
Alphas and Seledíë were many things, but small thinkers was not one of them. Especially since, you may note, they had very good reasons to build a capital that could out-impress everyone, everywhere, anywhen.
So returning to the general case, sort of like this:
(The above is an Nvidia test image, named “Complex at the Center of the Universe”, about which the TV Tropes page cross-links to Your Head A Splode. It’s worth clicking through to the full-size image and appreciating all the little details.
This is relevant, in particular, because it would be entirely accurate and unexaggerated to say that your modern Imperial city planner or arcology designer will look at this and thing, “Hm. Not bad. A little modest, but a good place to start.”)
1. And the reality behind those visible manifestations, of course. It would be hurtfully inaccurate to say that the great and near-great worked so hard to abolish poverty just because it offended their sensitive souls to have to look at it, or rather put up with the knowledge that it existed in their personal universes. But it was a nice bonus.
Holy City: For the Empire, Ellenith, the holy city which serves as the center for the Church of the Flame, being built around the Dome of Unity, the 6th of the temples along the Shining Path, the seeress Merriéle‘s path to her conversation with the gods atop the mountain Tirias Calémon. (One might also consider other locations relevant to her life as this, save for the small problem that her birthplace isn’t actually known, and the place of her death no longer exists due to the, ah, explosive circumstances of same.)
It is a particularly holy city, of course, because the whole city is one giant temple, constructed as a giant engraved celestial mandala with all secular functions banished to the towns and villages inside the Deeping, yet outside its precincts. While it does house some indirectly theological function, it’s clearly dominated by its temples and shrines (every accepted order has at least one here), its libraries, and by such other special hallows as the Vaults of Earth and Water, the Hall of Chained Gods, and so forth.
(Depending on how you look at it, you might also be able to consider the Deeping as a whole – fields, forests, towns, and all – as an example of this category: it’s located in an ancient astrobleme, and the entire rim has been carved into a labyrinth, with all manner of vaults, halls, and curiosities hidden therewithin, even aside from its defensive function.
The larger Ledges of the Dead almost certainly also fall into this category.)