Age of empires did a good job of the Aztecs, Mayans and Inca but often makes the game play for other neolitic or “edge of the world” people either very limited or mere NPC’s. Yes it is true that the North Americans, Australian, pacific and a few others did not make empires and castles but that was often an accident of history. Australia had all the resources of Eurasia but all the best resources were in treeless deserts. Similar problems are found elsewhere.
AoE IV wants to do Amerindians and has them with horses in a game with Rome and Japan, etc; a bit of creative alternate history is needed.
I thought of a solution that should not break the game style away from AoE 2, much, which is everyone’s idea of classic AoE.
Start these off without a stables as per the Aztec but:
Add in a dog travois for north american indians. https://en.wikipedia.org/wiki/Travois
Pair this with a hunter scout for your long range scout [eagle warrior scouting]. However if this unit defeats a mounted unit the dog Travois is replaced with a horse travois and if that is delivered to the town centre it can then build a Stables. The stables then produces the Amerindian mounted units. If the map has no civilisation with any horse units then none of the civilisations will have a stables. You get historically accurate play.
[Also In snow maps the Amerindians should be able to make a sled dog team that serves as a trader/ scout unit or fast gatherer scout. He becomes a horse sled on defeating mounted unit. This is also an option for the sami of the Eurasian Arctic coastline.]
For sub saharan africa; bantu, Zolo, etc. This mechanism would be a warrior herdsman with a big African bull in tow. Again he can capture a horse and on getting back to the town centre that triggers the stables build. He is not riding the bull.
For Australian Aborigines & Argentinian Mapuche: add a hunter scout with a dog. Same functionality. One or two horse units only from their stables.
The basic idea is a fast scouting unit with no bonuses against cavalry that must beat and capture a horse to get the stable technology. This unit is recognisable because it has a ‘pet.’
For the pacific islands its an out rigger canoeist that can capture more advanced ship technology. Horses on islands are of limited use.
Stables units: Amerindian: Mounted spear; Mounted archer (keep distance function); Fire arrow rider, Travois trader.
Aboriginal case: Mounted skirmisher & Bush Ranger (gun powder).
Mapuche: Mounted bolos & Gaucho facón or Gaucho rebel (gun powder).
Rather than have a castle age and castle build allow a simple name change flag for these peoples to Tribal Confederation. Add a defensive mound build: more food and less stone/ gold. This serves the castles function. Give these neolithic races a reduces gold mining speed but add trap lines that give them gold slowly. Give them a diplomacy victory condition: Great Treaty.
There are plenty of mods for assets such as tipi’s, tents, huts etc. As an Australian I have other suggestions to balance the aboriginals. There are a few historic unit options. (Contrary to most claims the aboriginals were not easily defeated in battle. They just thought better food and clothing was won by joining in and becoming farmers than fighters. Racism came later.)
This way we cover all the world without making the peoples out on the edge easy conquests or NPC’s.