Top 10 tips for everybody starting the game! (non-spoiler)

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Not sure if it is documented anywhere, but it appears that each point of Attack rating increases your damage by 3.3%.

So if your unit's Attack is 30 higher than the enemy's Defense, you do double damage (+100%). 60 higher, and you do triple damage. Apparently it caps out at triple damage.

Similarly if your unit's Defense is 30 higher than the enemy's Attack, you take half damage. 60 higher makes you take 1/3 damage. Basically each point of defense increases effective health by 3.3%.