Here's the deal in RTW, as research by the Fourth Age: Total War team shows:
The "chance to kill" formula goes like that:
Code:
chance_to_kill = dlf * const * lethality *1.1 ^ (ATK - DEF + BNS)
and the "chance to hit" formula is the same but without lethality. That should give you the analogy of atk and defense and also the info that each point of those changes the result by 10%.
dlf is the difficulty level factor. 0.7 for Easy, 1 for Normal, 1.5 for Hard and 2 for Very Hard. If you do the math you'll see that this is what gives the combat modifiers of -4, 0, 4, 7 to the AI depending on battle difficulty setting. The constant is some number, the value of which, though of little importance since it's fixed, is still a matter of debate. Two good guesses would be either 0.19 or 0.095, the latter of which gives a max 77% chance to kill on VH difficulty.
Reagrding animations, their speed matters as well as their impact deltas, which in the case of spears and 2handed weapons are rather bigger than usual.
Cavalry (and generally unit) mass has no immediate effect on damage, it does however help, because it gives more penetrating power to the unit and allows for better utilisation of the charge bonus. In proper melee, it just helps push the enemy, no significant difference...
When 'ready'/spears-braced the spearmen also negate and reflect the cavalry's charge bonus back to the cavalry, apart from their usual ant-cav bonus.
I don't know exactly what's the differences between RTW and M2TW, but their engines seem much closer than say MTW and RTW, so most if it must be the same or very slightly different. For example in RTW defense skill counts only from the front and right side and shield only from front and left side, while you say that in M2TW they both apply to both sides. Also in RTW the shield bonus gives protection equal to twice its value against frontal ranged attacks. I don't know how this works in M2TW.
Hope this helps.
Cheers!
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