Why Don't My Mages Cast the Spells I Tell Them To?

[Edi]

It also depends on what magic is available to them. If all you have is flying shards or fire darts, they'll use it on the militia, because then there is at least some chance of actually doing damage, whereas heavy cavalry is virtually immune to those spells (heavy armor + shield), but if you have a fire mage who can use fireball or flare, those heavy cavalry will start suffering rather immediately.

Edi