Starting ~level 5, you'll really start enjoying your priests. I'm considering running more priests just as a habit. You get enormous heals per day, which nets you a lot of combat potential. Even enemy groups that are like "you don't stand a chance" can be easy pickings with 100% autocast priest just keeping your guys topped up. They also get Haste, which lets your units attack more than once a round. When you've got powerful characters, extra attacks becomes super powerful.
Archers are pretty OP, because they attack twice, but warriors require no ammo and are much more durable. They make a very good party, altogether.
Mages... they're nice. They mainly give some decent debuffs, and at high level they can cast Magic Bomb thing that's useful for wrecking enemy cities. They also make good mayors, as you can just raise their INT only, then raise mayoral skills, and install them as mayors, only taking them out to siege castles or such.
You can ALSO inscribe spells onto blank scrolls, around level 8 or 10 maybe, not sure. You can have a priest put Haste on scrolls, or a wizard putting Detonate on scrolls, so you don't even have to take them out. I'm planning on getting a stockpile of Haste scrolls, as they're REALLY useful in general.