Yes, this is a difficult point to imagine : what are the limitations of magical power in such a campaign setting ? Ingame, you can almost do everything when you are a priest, a mage or any other spell caster. Druids can also use divine magic, that allows them to control climate, plant growth, and so on...

As I imagine the city, as a capital, it has to show the power of the whole nation, so spending much money and energy only to have fountains and grass in the middle of a hot desert could be not so eccentric. In my opinion, of course...

I'll keep building my map.

Thank you for your answers