Active Defense

Normally, D&D is played with active attacks and passive defenses.

The attacker rolls, compares to the target's AC, and then determines if he hits or not.

Alternatively, there's the "players roll all the dice", which I like for defending because:

• Players feel like they have more agency.
• It frees up hands and time for the DM.

But active defense is still less fun than attacking.  When you're attacking, you're deciding who you want to attack, how you want to attack them, coordinating with your allies, etc.  Fun.  While active defense still feels very reactive.  

"The ogre tries to split you like a log.  Roll Defense minus 4."

Better than the DM rolling for the ogre and telling you that you're dead, but still just just calculations.

Ideally, you'd want to bring some decision-making into the act of defending* so that it becomes about making interesting choices instead of pulling a lever on a slot machine to see how much damage you take.

<digression>*This is actually debatable.  Do players want to have to make decisions about how to defend from five goblin spears?  Or do they just want to get to their turn quickly so they can open the cage to the dire moles and see how that pans out?  To put it another way, can we make defensive decisions interesting?</digression>

I have one other design criterion: I want the new mechanics to be completely compatible with preexisting OSR systems.

Dodging and Blocking

So here's what I got.  You have two ways to defend:  Dodging and Blocking.

Dodging works exactly the way that AC does.

Armor and Dexterity improve your Dodge; and when you dodge successfully, you take no damage.

When you block, you move into the blow and try to absorb it via your armor/shield.  You take damage from the attack, but you reduce that damage by 1 if you're wearing heavy armor, and you reduce it by another 1 if you are using a shield.

Leather Armor: +2 Dodge

Chain Armor: +4 Dodge, +1 Block 

Plate Armor: +6 Dodge, +2 Block

Shield: +1 Dodge, +1 Block

When an enemy attacks you, you can choose whether to dodge or block.

The Math

I made a spreadsheet; It's actually pretty complicated.

Blocking reduces damage equally regardless of the opponent's attack bonus.  It's just a flat reduction.  Therefor, blocking becomes useful in two situations: against enemies that 1d6 damage, and against enemies that have very high bonuses to hit, who are probably going to hit you anyway.

<digression>I think I'm going to cap the attack bonuses for the GLOG at +10 for this reason, among others.</digression>

Blocking also does another very important thing: it removes the chance that you might get critted.

Since you're reducing the spread of potential damage, you're effectively buying insurance.  You're removing the chance that you'll take maximum damage by removing the chance that you'll take no damage.

(Which makes sense fictionally, too.  Crits happen when you're trying to get fancy, jumping around all nimbly-bimbly, and you slip in the mud and take bit hit to the face.  Whereas blocking, you're just hunkering down and taking the hit on your shoulder.)

If you know you only have to survive on attack from a goblin with a 1d6 sword, and you've got 6 HP, you can play it safe by blocking instead of dodging and risking that 6 damage hit (or a crit).

You also don't want to block if you're at 1 HP.  (Makes sense fictionally: you're an inch from death, too tired to raise your shield, just trying to stagger out of the way.)

HOWEVER, if you do the math, blocking usually sucks.  By the time you have a high blocking value (platemail + shield) you usually already have a high enough AC that blocking is inferior to just defending as normal.  Still, I think the choice is interesting.  (Other people may not.)  So blocking is highly situational; dodging is the better choice nearly all the time.

Can Monsters Block?

No.  Keep it simple, keep it fast.  To do so would go against the design goals.

Metal vs Wood Shields

If you wanted to differentiate the two, you could have:

• Metal Shield: +1 Defense, +1 Block
• Wooden Shield: +1 Defense, can be sundered to reduce incoming damage by 1d12.

Parry

If you wanted to have a third option, you could have a Parry option, in addition to Dodge and Block.  You'd roll your attack bonus to defend instead of your armor bonus.

Except I don't want level 5 fighters running around naked, getting better defense from their daggers than they would from full plate, so maybe a condition is needed?

You can only parry when you are wielding a weapon and defending against a weapon, wielded by a human- or halfling-sized opponent.

Criticism

So I just re-read all of stuff I just wrote and I hate all of it.  Normally, I'd delete the draft and go watch Star Trek, but I'm going to leave the post here because it's a good discussion.

I can explain.

Back when I played 3.5, I had a Power Attack calculator.  Based on the opponent's AC, and my average damage, how many points should I sacrifice for power attack?

I had fun doing that, because I felt clever, but now that's exactly the sort of gameplay that bores me.  It's math, instead of interesting choices.  If you know the numbers, it stops being a challenge and becomes a known solution.

So, although I just wrote them, I dislike the Dodge, Block, Parry mechanics because they're a known solution once the math is figured out.

And it's a boring problem because the two goals are directly comparable.  You're trying to choose the best way to minimize a single variable: the average damage.  Or in the Power Attack example: you're trying to maximize damage.

Fun choices come from choosing between to incomparables.  Like, should I attack with my sword (1d6 damage) or throw a molotov (2d6 damage, but is a single-use resource).  You can't boil that down to a single factor since you don't know how much you're going to need that molotov in the future.

The rule for sundering shields is potentially interesting, because you don't know if you're going to need that shield in the future.

Sundering #1: You can choose to negate all damage from an attack by breaking your shield.

Except that rule isn't that interesting.  In practice, people will keep the shield around, then break it in order to keep themselves from dying, and not a moment before.

Sundering #2: If you have a shield, you can choose to block with it instead of attempting to dodge.  Damage is reduced by 1d6, but if the base damage is 6 or more, the shield breaks.

That's more interesting.  Reminds me of the usage die, a little bit.  And it leads to some useful questions.

A bunch of goblins are attacking you.  Do you want to block the goblins with your shield?  It's numerically advantageous (in terms of minimizing average damage), but you run the risk of your shield breaking, when you might need it more later.

That's my final conclusion, then.  Block/Dodge/Parry is boring because it's a solved equation (that favors the spreadsheet-equipped munchkin) while Sundering Rule #2 is actually pretty interesting.

Need to playtest it.