I really like the crit system I have for Spells and Steel - getting a crit means you've gotten an advantage in close combat that will let you do something nifty, like disarm, throw, or hold the enemy.
But I'm moving away from the idea of Fortitude checks at the moment, and away from armour as damage reduction.
With that in mind, I think I'm leaning towards exploding damage dice in addition to the existing system.
A roll of 6 on your damage die means you get to roll another damage die - keep rolling as long as you get 6's!
Your first roll goes to the foe's Armour HP, and the second and subsequent rolls go to the foe's normal HP.