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Haven... What can I say, they are warriors, and that's what they are. Personally, I don't like them very much; I'd put them in 5th place out of 6. They are slow, and they focus on defense.
Peasants/Militia - in my opinion, there's no need to upgrade them at all, since militia don't pay taxes. And the retaliatory strike ability is used very rarely. Besides, that strike is weak.
Archers/Crossbowmen - you can use both in your army at the same time, since archers attack in an area, and they have higher initiative.
Swordsmen/Plate Armor - they are slow and focus heavily on defense. They are not used in the late game.
Griffins - they are quite weak, although they do have the ability to retaliate. Besides, they are large. However, the "Dive Attack" ability is good against enemy archers (especially against succubi).
Monks - they hit hard, and fanatics can also cast spells. Overall, they are not bad.
Knights/Paladins - they are essential in the army. With their cavalry charge, they are almost unmatched. The "Lay on Hands" ability is also good.
Angels - they are fast, but the ability to resurrect is especially valuable. You can use angels in pairs with knights, supported by archers and monks.