No real city is a monolith, but too many fantasy cities are cut from whole cloth - just a bigger town, plus a bazaar or a slum or something suitably urban. I say this as much in admonishment of my own efforts as a callout for anyone else's. I grew up in a city where digging tunnels for the subway unearths Roman remains. No street matches its neighbour - there is always something so new that it's still being built, while some houses are older than many modern countries. Some trees are older still. On one of the hills near my old school, a queen was said to have rested beneath an oak and thereby blessed it - the tree died centuries ago, and in its place is a foundation for an anti-aircraft cannon, grown over with age, now a hangout for teens and the homeless. There is another hill that is rumoured to be not a hill at all, but a mass grave of bubonic plague victims, piled high and composted by time. A freeway cuts across it, and the field often plays host to a circus. That's a city to me. Let's do better, shall we?
Write 6 important places in your fantasy city. An entire area can be just one of the six, but specific major landmarks get their own entry. (If you do more, change the die you roll for the next steps to match.)
Roll 1d6. Fate or the gods or simple luck smile upon that place in its current form, for good or ill - draw a little symbol next to it. Do this once or twice, or more if you want. Symbols can stack.
Now, roll 1d6. Cross out the entry you rolled (unless it has a symbol, then cross out the symbol). When you cross something out make sure you can still read what it once said - then replace it with something new.
Inspiration for what the new thing is and why the old thing is gone can come from a reading of your GM's Oracle, or these tables:
Scale of upheaval:
Nature of upheaval:
3. Patronage of the arts
4. Steady political progress
Keep going until nobody alive could remember how and why all these changes happened.
Forget all the crossed-out bits until they become relevant to your players