Here's my 2 cents on the new building system from EE, specifically iron.
I'm not a fan of how acquiring a region with iron only lets actually use that iron in that region. Province layout is already restricted as it, with limited building slots (not saying that's a bad thing!). It just doesn't make sense that you can move marble halfway across the world to build a temple, but you can't move iron one province away to give someone a better sword.
I understand CA's intention of making certain provinces more important, but I think that's still accomplished by how resources like marble are handled. Control the province, and you have access to it's resources throughout your directly controlled territory.
Horse upgrades being restricted might make more logical sense, as I guess it symbolizes a region being known for it's equine pedigree.