I think there's a clear division between primary units that require resources, and secondary units that don't, but I don't necessarily see the resource-free units as "defensive." Musketmen have a good basic strength that is useful both for attacking and defending. Archery units are going to be just as important on attack as on defense.
Valid points.
I guess one's playstyle influences one's view of units. It is absolutely true that many units are multi-purpose, but I am looking at the units that require resources and seeing mostly offensive units (e.g. units that require horses and siege units often get no terrain defense bonuses) There are some good multi-purpose units in there also (e.g. Longswordsman) but they are expensive and a larger force of weaker but well-positioned defensive units should still stand a good change at holding them off, but not at attacking them.
Given a hypothetical situation* where equal production leads to 4 longswordsmen facing off against 5 musketmen:
If the longswordsmen are on offense it is a pretty even fight, probably favouring the defender especially if they are fortified in good terrain.
If the musketeers are on offense I suspect the longswordsmen have a slight advantage even unfortified on open ground.
So I would assert that the 'secondary units' are sufficient for defense but "suboptimal" for offense.
Still, I don't see the bulk of the resource requiring units (we desperately need an agreed shorthand for these terms!!!) as primary, I see them as primarily offensive.
So its more a case of 'I won't miss large numbers of the those offensive units if my strategy is defense' than 'I cannot play offense if I have few resources'.
Which goes back to Calouste's statement that "you no longer lose the game when you have no strategic resources near your starting location.", it's just going to be harder to win a pre-1000AD domination victory
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*I know this is unlikely in reality because a variety of units will be used, but I'm not sure how else to directly compare them