The popular belief for the casting order is the following:
All units from all nations are ordered by a unique number. Rituals are cast in this global order, but it is random each turn whether this order is processed highest to lowest or vice versa.
Example: Assume there are only four mages in the world. So either they cast A B C D or they cast D C B A, but B will never ever be the first or last to cast.
However, due to the high number of mages and the fact that not everyone is casting a ritual each turn, one may say that practically rituals are casted in a random order.
Note that this global order is also used to sort the commanders in tactical setup. It also determines the order in which the mages of the same side cast their spells on the battlefield during a combat round.
One may exploit this by ensuring that some spell X is cast before spell Y:
- In the battlefield, script a mage on the top with spell X and on at the bottom with spell Y.
- For rituals, one has to cast X twice: Once with a mage listed before the mage casting Y and once with a mage listed after. However, I have no idea were this is useful (e.g. lowering temperature with Wolven Winter only takes effect after all spells are cast, so simultaneously attacking with a cold-based distant attacks in the same turn will not benefit from the wolven winter).