The population increase for units needs to be handled with caution.
At least the only units with comparable population power levels are the Gilded versions of the Order of the Dragon and the War Elephants.
- Knights.- Regular knights, for example, should cost 1 population.
- Cataphracts.- However, cataphracts are as overpowered as Gilded Knights, they should perhaps cost 2 population each.
- Ridari.- This one is curious: it lacks a charge attack, but it compensates with higher attack stats and HP. Cost the same as Knights in convertion cost. What truly makes them overpowered is that the mechanics of the Varangian Arsenal allow them to have +3/+3 armor and +15% HP in Castle Age (III). Technically they are as strong as French knights with Early Royal Staples Tech. Maybe, their base attack could perhaps be reduced slightly, but I don’t think it requires more population.
- Mangudais.- They are very difficult to kill in the Post-Imperial era, and yes, more ways to counter them should be considered. In the Feudal era, they can be countered with more archers, but in the Post-Imperial era, no; generally, you need either many Mangonels and some units that act as bullet cushions (Knights, Horsemen).
→ Historically, the counter to horse archers has always been foot archers. Age of Empires III knew this, and that’s why that’s their counter. Consider giving Foot Archers an extra bonus against horse archers (+1/+2/+3/+4).
—> The other option would be to lower their HP, which is currently too high (II: 85 III: 105 IV: 120), considering that other horse archers have less HP, such as the Rus Horse Archer (III: 85 IV: 100) or the Onna Musha (III: 80, IV: 95).
- Camel Archer: This is theoretically a living tank, and in the game, it’s represented as a “support” unit that also attacks at range. Its HP in the Feudal Age is so high and increases so much over time (II: 150, III: 175, IV: 200) that I think it should cost 2 population. However, if it’s going to cost 2 population, they could increase its HP a little.