The last of my playthroughs was a single city challenge. I noticed some design flaws that I think are interesting to point out.

1. Unit teleporting within city is very easily abused. I connected all my territories with districts, and then I can have an single army where within the city and defend it from attack anywhere, with full reinforcement from my whole city.
2. Food stacking can not exceed 1 population per turn. As in example from screenshot I need only 109 food to create one pop, as I produce 1211, over 90% go of my food go to waste.
3. District costs behave strange, at some point in the game I think I could build several districts per turn. But at the end as seen in the second screenshot the cost seams to scale more then my production since they cost 2644 and I can barley produce one per turn.

Yeah the scaling is weird, the gold cost seems to scale with production or something too.

I had the same building cost more depending on in which city it would be build, and each turn the price seemed to go up. I don't get why. Gold prices are very expensive too, everything costs tons of gold even early on. That's not a good way to "balance" gold.

Gold income should be balanced to not get ridiculous gold incomes too easily, things should not be extremely expensive, and gold price should also not scale like it does.