Under rating threshold (hideshow)
The highest value upgrades that make the most difference are: The last level of reinforcements (they throw javelins, making them basically summonable rangers hideouts, place them behind your barrack soldiers because their bows do a ton more damage than their swords), the first level of raining fire, which only costs 1 star but doubles the power of your raining fire. Almost every upgrade in the warriors tree, because that extra health for blockade becomes so much more important later on. Archer upgrades seem to have terrible value for the stars they cost (10% cost reduction? a rare chance of firing 2 arrows at once, at most buffing dps by like only 10%?) but it may just be preference.