1. to create a floating city you must learn/research the "Buoyant Earth" spell
2. pick a city to cast it on.
3. the cost of the spell rises exponentially (something the people helping the demigod team should get) with the city size. It should cost both mana and essence
4. Its should also cost mana to both maintain the cities place and to move the city.
5. If the Channeler can not or chooses not maintain the city or the spell is negated, the city will crash down onto the tiles it was suspended above.If its above land, it badly damages/destroys itself (depending on the tile) while crushing practically everything underneath. If its over sea, the city becomes an artifical island?
6. the city will need a portal connected to earth or a similar device to receive resources - else the denizens of the place may starve.
simply having the city 'anchored' to the ground by massive chains or similar, acting as bridges. By massive, I mean truly, enormously, enchanted, massive chains. Enough to travel along, to and from the city.
When and if mobile, the chains are unhinged and retracted, severing the city from any and all support. It cannot send or recieve landborn caravans and cannot harvest out-of-city fields. Essentially, the city will inevitably starve and run out of resources in due time.
7. The city can only directly be attacked by spells and flying creatures/people dropped off by flying creatures