First off Song dynasty is the most impactful dynasty because it’s the cheapest, you can get it 5 to 6min into the game and it give you access to long term eco by way of faster villager production and granaries. Also being that you no longer LOSE access to unlocked dynasty units and buildings effectively after say 20 minutes of song dynasty double TC, you would have exhaust the full functionality of song dynasty (easily 100+ vils, granaries, zhuge nu unlocked).
Now let’s consider the current state of Yuan Dynasty? The 15% speed has virtually zero economic impact if you already have wheelbarrow, so the speed boost is strictly military, IO and villagers running away from danger. Next in a vacuum fire lancer are niche units with decent function, but they aren’t enough to merit going double castle the way CURRENT Grenadines are worth going double Imperial. Next are pagodas which are DECENT buildings IFFF you can secure 3 relics but again this building by itself nor along with the other perks justify double castle in a 1 v1 match.
I think the issue with yuan dynasty currently is NOT the cost but the IMPACT!! for 1 if you went for an optimal yuan dynasty timing you’d be cutting your song dynasty boom short of 100+ villagers. You could legit hit double castle by 15 to 18min into game. But 15-18min is where most other civs have established themsleves and are nearing some type of power spike and that’s NOT the ideal time for china to effectively be resetting their momentum. Yes some ppl have suggested reducing the cost of yuan dynasty but i rather it remain costly but have a higher impact and here is how:
You can only make a single pagoda that auto generates 75 food, wood, stone every 30 seconds. However for each relic you put in it, up to 3 relics, you get an additional 50 gold per 30s and 25 food, wood, stone every 30s. The idea here is a castle upgraded villager is worth about 50 resources per minute and the pagoda is worth 150 wood, so essentially going yuan dynasty instantly gives you access to a 9 villager’s worth of production for the cost of 150 wood!! Then adding 3 relics gives the standard relic benefits but also 3 more effective villagers per relic for total effect of 18 villagers on food, wood and stone along with standard relic returns!!! This would displace the loss of song dynasty boom!
Permanently unlock the ability to make 2 more Imperial officials. This would synergize well with superivising 2 large mining veins meanwhile supervising your other major income sources AND still be able to collect taxes. And it allow for an even stronger emergency surge of units with 6 production building supervised to effectively be 15 production buildings!!!
Next the Imperial Offical get a wheelbarrow upgrade that takes 30 secs, can be researched from the TC or imperial academy or spirit way. Gives IOs 15% movement speed, 100 more carrying capacity and… 20% bonus on tax collected!! So you’d have an IO collect 80 gold from a mill for example and turn that into 96 gold!! And to address the issue of inefficient capping, the IO will always properly apply the 20% bonus even if it means it has to go over the displayed cap of 140 or 180.
Fire lancers… now in the PUP notes they said they got something in store for them… so we’ll see, but meantime this is my suggestion for the unit as a raider. Bounty!!! Yes give the mongols bounty system to the fire lancer. I’m talking 50 food 50 gold. Make it so if the fire lancer tags the building at anytime and subsequently that building is destroyed, the fire lancer gets the bounty. So now you can make a small raid party of say 4 fire lancers that can rake threw congested woodline, killing easily 4 to 5 villagers then instead of chasing down that 6th villager can turn around and burn the lumber camp and get the bounty. Then run away to raid another exposed camp!!
I believe no one mentioned perk above would be worth double castle even though each are exceptionally improvements compared to current. But the sum total would maintain Chinese castle momentum and be worthy risking the exposure to not having equal army and losing song dynasty privileges. I dare say it might be worth to skip early song dynasty and go fast castle lancer harassment into yuan dynasty!!