And let's not forget the wonderful spell
Army of the Dead. For just 10 death gems you can plop an army of 40 longdead (plus a longdead commander) somewhere as an instant reserve. In places that have seen some decent combat recently, you can also get an additional up to 20 souless (plus a souless commander). These 62 defenders, added to a PD of 11 or 21 tend to discourage the AI from attacking that province. If the AI masses enough forces to threaten the province so defended, you'll have the great joy of then being able to use Fires from Afar and other mass-slaughter spells to disrupt the upcoming festivities. Not to mention the various commander-killing spells.
