What does the troop-type Defense research do?

{Cavalry/Archers/Spearmen/Shieldbearers} Defense reduces the damage taken by your troops of that type:

• {Type} Defense I: −0.8%/level damage taken.
• {Type} Defense II: −2%/level.

It's the defensive wing of the War Tactics tree, mirroring the Damage nodes.

When to prioritize it:

• You defend a lot (seats held, garrisons) → defense offsets attrition.
• Otherwise, in pure attack, the type's Damage usually comes first.

As with damage, focus defense on your main type — spreading it over 4 types dilutes the effect. One type maxed beats four half-done.