Probably have to start with some great empire that conquered and vassalized most of the world then fell to get the physical and magical infrastructure.You'd have to account for that in society, though. Farmland would have to have immense walls surrounding it, and that would mean a smaller great wall of China around the miles of farmland and villages surrounding surrounding each city(or maybe it's one great city). Otherwise cities wouldn't be viable, because the populations couldn't be fed. Travel would be virtually non-existent outside of teleportation in-between cities, and that would be controlled so that monsters with teleport couldn't just appear that way.
It would be an interesting setting, but would take a lot of thought to set up properly.
Adventurers would be sent out to deactivate magic circles in fallen cities so monsters don't get in the network or execute mega monsters that threaten reclaimed cities.