No problemApologies for a delay with the answer.
The idea of the copper resource for building of early defense units is pleasant to me (I thought of something similar). A problem in too big "step" between the defensive force of units, not demanding and demanding copper. The defensive force of Tribal Guardsman - 2, Spearman - 3. The difference by one and a half times, it is a lot of. Also at a significant amount of the nations in the place of start it can not turn out in availability of this resource that considerably worsens their chances of survival.
I have no exact recipe of the solution of this problem. But there are some thoughts...
The way is radical, but labor-consuming - it is possible to double force of the attack and defense of ALL units. Then Tribal Guardsman will have protection force 4, and at Spearman - 6. It will allow to enter between them one more intermediate unit (which is not demanding copper) with a defensive force of 5. Naturally, it will give the chance to add, if necessary, and other intermediate units, having increased their variety.
The second option - to make units doubles of early protective units (Spearman, Pikeman) with the requirement of a resource of iron, perhaps, with the raised building price.
Ok, but for now i cant make such a big changes, because of luck of time.
If you will make such a thing, post here what will be a result.