I've done some thinking about it, and post my latest round of thoughts here.
*Note General assumption is that each tier of weapons are/will be designed to be used against latest armor. (IE like spears, axe, dagger, warhammer vs. leather. )
** Leather armored target is going to have 12 DEF , 2 more with latest shield, 5 more from defending, or 19.
** Chain armored target is going to have 22 DEF, 6 more shield, +15 defending, or 43.
** Plate armored target is going to have 56 DEF, 8 more shield, +20 defending, or 84.
*** Next thought was to make weapons to hurt/damage armored target w/o taking defending into account. (The bonuses seem huge, could be looked at to be lowered some)
1) Spears, Warhammer upped to 12 ATT(ack).
2) Axe upped to 13 ATT. Differentiation.
3) Dagger upped to 8 ATT. Differentiation again.
4) Shortbows upped to 12 ATT
5) Boar Spear upped to 16 ATT
6) Longbows upped to 32 ATT
7) Throwing Knives need to be adjust A LOT damage wise which doesn't seem likely, so next best thing would be to kick back to Training or Weaponry techs. They're going to do nothing against chain. Not a whole lot vs leather either, but at least they can be acquired when they're somewhat useful.
8) Mace upped to 32 ATT
9) Short Sword upped to 30 ATT
10) Battle Axes needs 36 ATT
11) Long Swords 48 ATT
12) Need a pierce weapon same tier as Longbows/Longswords (Longbows and War Colleges techs ) with same 48 ATT recommendation.
13) Pike, Greatsword, and Maul upped to 64 ATT
14) Need Axe to fall in with weapons in #13, and 72 attack ( axes are 2h and do more damage )
15) Magic versions of same weapons should have same base rating as weapon of that type plus doing whatever, extra fire damage, easier to crit, etc.
16) When it comes to Elemental Damage type weapons I suggest something like +8 bonus early (dagger/spear/axe/warhammer/staff ) , +16 midgame , +32 late game.
*Note largely looked at this issue with the idea was going to leave armor alone, but the following issues stood out
1) Largely thought about leaving leather alone
2) Chain mail should offer a small bonus towards piercing damage. Cutting or edged weapons will lead to cutting of the chains, reducing effectiveness and exposing a weakness. So this is counter to how chain mail works. The bonus should be +2 per piece, save for greaves and breastpiece which get +3. (Chain Greaves and Breastpiecs have defence stat of 6, not 4 like the others. )
3) Plate Mail should offer a small bonus towards cutting / piercing damge. The same idea, bonuses being +4, +5 for greaves, and +7 for breastpiece (since armor stats are 8, 10, and 14 respectively) Also interesing is Blunt damage is how you take people wearing plate out of battle.
If you want to code in penalty vs damage types be my guest. But I'll settle for reducing the bonuses and correcting types. For example: consider a defensive type unit wearing all chain armor, mace and kite shield. 28 DEF. (Let's seperate the Kite shield out of this, so 22 DEF. ) Say you made chain have a penalty and it's 50% less effective vs cutting / edged weapons. Instead of 22 Def from chain, he's going to get 11. (But he'll still be able to use his shield ). So a Mace or Bow would work against 28 DEF, while versus sword 17 DEF.
4) I don't think ignoring % of targets defence should work when its piercing damage up against plate mail. Arrows are gonna bounce, spear shafts will break.
Is it perfect, no. It's a start at least to get something working, without having to completely re-balance armor too. Hopefully spurs discussion