OK, because I have too much time on my hands today (and I really like making these little things), here is a battle plan!
But first, something else I was thinking of (and, yes, I'm giving it way too much thought, but as I said, too much time).
Strategy--Why are we going to war? What is the goal of a war against the Inca? My impression is that this is a war for resources and breathing space. If the goal is something else, correct me because that may change the overall plan . . . . However, using this as a basis, we should probably not commit to wiping out the Inca in one go, but clear out our border and strike their core.
Also, if the scope of our war is limited, we may not even want to attack their distant cities, like Vilcabamba and Scithian, because they will still deter the other AI from settling in any vacancies we leave (and we can take them later).
Anyway, the battle map:
Lemme break it down.
Purple Group: 6 Horse Archers, 2 Archers, 1 Axeman. The Horse Archers will strike and take Tiwanaku on turn 1 (assuming there are no more than 3 defenders). We may want to have a couple of Flanking units here. The infantry will HOLD BACK until the city is taken, which should allow us the use of the nearby roads to get the Archers in town on Turn 1. Purple Group will then heal.
Green Group: 2-3 Mounted Units (Chariots/Horse Archers), 2 Archers, 2-3 Axemen/Swordsmen (but at least 1 Axeman). As indicated, they will take the Incan town on Turn 1. I would personally raze the city, unless the team thinks that there is some value in keeping it.
Black Group: Composition unknown. These are basically going to be the reinforcements that we assemble after hostilities are commenced.
Once Purple and Green Groups are healed, they will advance (save any defenders needed for captured cities) to the indicated location. If Black Group has been assembled, it will advance as well. Each team should have at least 1 Axeman to deal with Incan counter-attacks.
Blue Group: All groups merge for the siege of Cuzco. As of now, the defense of the Incan capital is unknown, but we should plan for at least 6 defenders with some promotions. Even with 50% losses from Purple and Green Groups, we should be able to put together at least 8 units, maybe more if we can crank them out.
Yellow Group: This group will probably not exist. I would like to know what tile is east of the Blue Dot. It looks like water, but in the event that there is a land route to Yellow Dot, we should put at least and Archer and an Axeman there to prevent Cuzco from being reinforced along that land bridge. Unfortunately, it looks pretty clear that we can not reach that tile, so this part of the plan will probably be scrapped.
Other Note: We will need roads leading to Green, Purple, and Black Groups' starting locations so that the attack can commence smoothly and so that reinforcements can quickly be brought up to the front lines.
A small garrison should remain in St. Petersburg to deter any Incan counter-attacks from the north.
I do not know if the situation has changed, but the Incans do not appear to have access to Horses. Therefore, we should not need to build Spearmen for this attack.
After these three cities are taken, we should take peace and do our best to use our new acquisitions to enhance our economy. Once our financial situation is better grounded, we can continue our aggression against the Incas or whoever else poses a threat.