The One Built City Idea:
Difficulty: Emperor
Map: Small/Temperate/Flat/Pangea/ 20% land with optimal # of cities for corruption changed to 12 from 14 (so we can build the FP -see below)
Opponents 5 - No scientific Civs, otherwise random or maybe handpicking civs with low cultural priority (Rome/zulu come to mind)
Varient Rules:
We may only build One city ever.
We may obtain up to 5 more cities maximum (enough for fp), but must propaganda/flip/capture them, they must be capable of joining boarders with one of our current cities on first boarder expansion which must be done asap by rushbuy or whip of a culture building, even if it isnt ordinarily a prudent thing to do, and the city may work no citizens until it has a trade route and joined boarders with the capital.
Once we take ownersip of a city, it is our city and may never be abandoned or replaced. If it is razed, then we just have to plug along with no more than 5 total cities the rest of the game. If it is captured, we must make every reasonable effort to recapture it.
I know this doesn't sound too tough, but the kicker is that we are going for space ship victory, with all but culture enabled. (Might be impossible on Dieity with these restrictions, which is why I suggested Emperor or maybe emperor plus with a 70% cost factor for the AI).
I have won this on Monarch, so it is doable on that level. I did have a pretty ideal start though. It would likely require a good, but not overwhelming start. Food bonuses wouldn't be a big item, but a river and costal access (so we could build the Colossus) and a non jungle wasteland area would be a huge help.
Civ would probably need to be scientific.