Will it be possible to mod in a defense stat?

[Spatzimaus]

I wouldn't be surprised if this is something that wouldn't even be possible. It sounds like a "core civ engine" thing to me

Not sure I agree with that. We know they have (from the pre-release info):
+X vs melee units
+X vs mounted units
+X vs cannon
+X vs armor
+X city defense
+X defense in (terrain type)
Doesn't receive defensive bonuses
Can't attack

Between these, I think you can get anything you need. For instance, to make a Civ3 Musketman (2/4/1), make a unit with power 2, +100% city defense, +100% defense in hills, +100% defense in forests, +100% defense in plains... you get the idea. Since I'm sure they've seen the appeal of that and it'd actually be easier to code, I think it's pretty likely that there's a flat "+X% defense" built into the engine, even if it's not used.
And unless it's hard-coded to specific numbers, I don't see why you couldn't put in negative percentages to create offense units. Cavalry would be 6 power, -50% defense.

But the real question is, why do you WANT a defense stat? The situational modifiers are much more accurate, besides making a better game. Take, for example, the Pikeman. Civ3 Pikemen were obviously much better at defense than offense. But once gunpowder (or even longbows) became common, they should have been almost as weak on defense as offense, because the whole reason for their "good" defense (long spears that keep the enemy at bay) ceased to be effective... except against cavalry. Which, coincidentally, is what Civ4 has them as.

The only real motivation I could see for separate attack/defense numbers, in light of this new system, would be for an Alpha Centauri-style mix'n'match Workshop system.

 

[WildWeazel]

I didn't mean you couldn't simulate it, just that you probably won't be able to completely rebuild the way combat works by reverting back to Civ3 style.