Working out armor for "Spirit of the Sword" is considerably easier than working out weapons. Actually, it's easier for having done weapons, since they kinda go together.
Like I said in my last post, I didn't want armor to just soak damage. A million games already do that, and it doesn't especially take advantage of FATE's strengths. My solution is to have armor work essentially just like weapons in reverse, using Aspects, adding Health Stress boxes (instead of adding damage), and, for top-of-the-line models, allowing the wearer to withstand an additional consequence (the mirror image of the pay-a-Fate-Point-to-inflict-a-consequence utility of the greatsword, war maul, and greataxe). There are three types of armor: light, medium, and heavy. I don't see a need to get more detailed than that. Wearing heavier armor also means taking a penalty to Athletics. I realize that this could be modeled by tagging an Armor Aspect, but my fear is that doing so would encourage, rather than discourage, use of Athletics. A fella could build up quite a store of Fate Points that way.
Unlike any other Aspects, Armor Aspects can be invoked once per scene for free. After that, it costs a Fate Point, like usual. The "Armor" field on the character sheet will have a check box beside it, a la FATE 2.0, so it's easy to track. When you invoke your armor's aspect for the first time in a scene, check the box. Admittedly, this makes armor a little more fiddly than just about anything else, but I think the system can take it. Invokes are pretty intuitive in this context, I think, but every armor type also has its weaknesses and strengths, and can be tagged in the former case and invoked in the latter. Armor Aspects are usually just the name of the armor itself: Plate Mail, Chain Mail, Soft Leather, etc.
Light:Soft leather, cuirboille, mail shirts, gambeson, etc.
Strong Against: None.
Medium: Chainmail, scale mail, lorica segmentata, brigandine, cuirass, etc.
Strong Against: Slashing
Special: Adds one Health Stress box. Imposes -1 to Athletics.
Heavy: Half plate, full plate
Strong Against: Slashing, Piercing
Special: Adds two Health Stress boxes. Allows wearer to take a second moderate physical consequence. Imposes -2 to Athletics.
I picture the Health Stress track as having, essentially, two rows: one for your own Health Stress, and one below for your armor. That ought to keep things clear. Like so:
Health:     
Armor: O O
Shields, on the other hand, give a +1 to a defensive use of Melee, or when using Melee to perform a Block, and make the "full defense" action give a +3 instead of a +2.