Something i just discovered from watching Marbozir's Youtube vid is just how powerful promotions can be in the case of China's unique unit, their crossbow replacement, the Chu Ko Nu. They're doubly interesting because it's a renaissance era unit, giving you the option of expanding then rather than waiting for +3 range artillery, or even better, bombers, to capture cities.
What is a Chu Ko Nu?
It's like a Crossbow, with the same 2 hex range, but it is able to fire twice per turn. Each attack hits for about 30% less damage, but with two attacks ... do the math!
More importantly, when upgraded to Gatling and then Machine guns in the renaissance era, they retain this two shots per turn characterisitic , where those units normally only get to fire once.
But the real reason for loving Chuk's is that they get XP twice per turn, because they can fire twice per turn. This means you can get access to high level promotions you'd never normally see - IE. the +1 range promotion - while still in the renaissance era.
With the +3 range and the ability to fire twice per turn, you have no need to build siege units!
My Tips for Chuk abuse -
1) Make sure you take the Honour policy that gives +50% XP in combat.
2) Build archers in the Ancient era and roam the map hunting Barbarians with them , to get some initial XP. After level 3, and 30 XP, you stop getting experience from fighting barbarians, but it's a good start.
3) Upgrade these units to composite bowmen, use them in any early wars you get into but above all else, keep them safe, you don't want to loose all those hard earned xp! Withdraw them if in any danger, and sacrifice other units to save them if needs be.
4) Make Machinery a priority tech, since it unlocks the upgrade from composite bowman -> Chu Ko Nu
How to Spend the XP
The all important +1 Range upgrade becomes available after you've maxed out either the ACCURACY (+damage on open tiles) or BOMBARDMENT (+damage in hills, forest, jungle tiles) upgrades. These max out at tier 3.
So, when your ranged unit gets it's first upgrade, pick either ACCURACY or BOMBARDMENT, and then stick with that same upgrade for the next two level ups. This means when you level up a fourth time, it is not possible to choose a higher tier of the damage promotion so the game offers +1 Range instead.
Don't do what i just did, and scatter the upgrades amongst cover, accuracy, and bombardment, then wonder why my level 7 units still couldn't upgrade range.
Please note that at the same time the +1 Range upgrade appears, the "Logistics" +1 attacks per turn upgrade appears. Unfortunately it has no effect on this unit , though it might be nice to have on a normal archer. Chu Ko Nu can never fire more than twice per turn, and when upgraded to Gatling guns they always fire twice per turn (unlike normal Gatlings which can only shoot once) whether you chose this upgrade or not. So, for subsequent upgrades pick something else.
I tend to pick "March" (also not available at lower levels) which means units heal even if they've moved or fired that turn, and put some survivability in with Cover.