Even in fantasies that read like a travelogue and science fiction that spends most of its time moving out in the black, cities tend to stand out. And why wouldn’t they? One needs to go there at some point. That’s where the supplies are, where the interaction is, where the political powers meet. Some stories, some games, never go beyond the bounds of a city, save a few flashbacks; a really big city is the gameworld in miniature, all the clashes and secrets and crazy concepts without the obnoxious travel times. But to get to cities like that, we need to start with younger places. More recent ones. Every large, old city is just a young city that’s been built onto and changed hands a bit more often.
With a young city, the best thing to do is to start with the overall structure; the people can come later. Consider it the skeleton.
The first question to ask yourself, then, is what sorts of people built the city. Most cities tend to reflect the aesthetics of the people who build them, both in how the buildings are put together and how the city itself is structured—is it made from a plan? Patched together at seemingly random? Radiating outwards in a pattern from certain structures?
Where is the city? This is going to become important both because of how the surrounding area shapes the city (there is only so much you can do with a valley surrounded by sheer cliffs, after all) and because of the limitations the environment necessarily puts on the materials available.
What materials were available to them, and where did they get them? This is going to determine just as much about the structure of your city; it doesn’t matter how much a culture loves using wood if it’s out of range of any forest, or how much they like to build of stone when what they have close by is mostly sand.
What sorts of special techniques, magic or technology did they have available to them? Most of what this does is serve as a way to work around the limitations created by the earlier questions: a civilization with roads and strong wheeled transports can work around the matter of being far from the forests, and one with magic that can turn sand to larger sections of something glassy could make building materials even if all they had was miles and miles of granular silica.
How do they get the materials they need to fulfill the city’s purpose? Part of this is survival—any city will need food and water for its inhabitants. Then there’s what the city was built for, if its location was chosen for any reason other than “there’s farmland and water here”: whether it was built to exploit a certain resource, to block an enemy’s vector of attack, to serve as an economic rival to an existing city.
Figuring these out may not get you the feel of a younger city, but they’ll certainly get you what it looks like—then you can build on that.