Celtic Heroes

Hello guys. I'm a new player and I recently bought double attack. Now double attack doubles your auto attack dmg but how to you level up your auto? Is that strength or dexterity?

Strength increases the damage multiplier of physical damage(slash, pierce, crush). The effect is diminishing, meaning the more strength you add the less valuable each point in strength becomes. Having 500 strength is better than 100, yet not quite 5x better because of the diminishing effect

Hope that helps clear up some confusion