@Eddit : for the ressources, well i was thinking to gems and coal, or something like that for the gnomes and wizards & co that rule this city.
i'm not sure how IA will Handle this. i have prob to adjust game parameters for this unit. Strangely, when the city fly next to a barbarian, if you don't attack and let the barbs having the initiative, the flying city is killed without defending itself (but if you attacks, all goes well).
I think it's because of the way IA Air strategy is handled. I put the city as IA transport.
If it's a civ prob (and not me who have missed something), one solution is to make it invisible (it fly so high in the sky that ...)
Anyway, here's an exemple of who you can expect to meet in the seas