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.
Quirky Town: Outside the arcologies, they’re all quirky towns, just about. Most of them, as you might expect, were founded by people with ideas. And then the people with ideas had memetics to help reinforce community and the local idea, which then gets to interact with the cast of eccentric individualists that any town in the Empire has as its population, and then you get…
(The arcologies, incidentally, are also pretty damn quirky. They just aren’t towns.)
Futuristic Pyramid: The pyramid is a popular shape – albeit sometimes adorned with side-decks, spires, bridges, etc. – for arcologies in particular, and also other large buildings, both for convenient load-bearing purposes, and inasmuch as it leaves plenty of room for gardens and balconies at all levels.