Tower Defense Heroes
Some heroes (e.g. Harold, Kris, and Ophelia) have Tower-Defense buffs. In most cases, you only need one of them: Harold.
Melee-Attack Heroes
Melee-attack heroes (heroes carrying swords, clubs, or other physical weapons) can deal close-range AoE attacks. They should be placed at U-turns where invaders will slow down so that the Heroes can deal more damage.
Range-Attack Heroes
Range-Attack Heroes (heroes carrying bows, magic staff, shuriken, or other range weapons) can deal single attacks with unlimited range. They should be placed as far back as possible.