I think some effects of the unique tech can not attract the player to build a castle specially for researching it, or even if the effect can help the player’s strategy, it is hard to research it in the time in need.
1. Ironclad: the effect that Siege weapon melee armor +4 can share to allies.
Just make it more worthy.
2. Crenellations: any garrisoned melee units can shoot arrows, not only infantry but also cavalry.
The occasion putting infantry into buildings is too rare, most of infantry like Pikeman are not worth doing this. Now you can put injured Paladin into the castle after skirmish, especially you have free Herbal Medicine and more arrows shooting.
3. Orthodoxy: not only gives Monks +3/+3 armor, but Boyar (or all military units) resist conversion +30% effective.
When Slav player go monk rush or siege monk rush, it’s hard to build a castle. Give it extra effect to make it more worth researching.
4. Corvinian Army: switch into Imperial age, make Magyar Huszars can be trained at the Stable instead of getting goldless.
The player can use cheaper common Hussars when he/she have no gold, or use stronger Magyar Huszars when he/she have good economy. Both of Magyar techs are useful originally, btw.
5. Recurve Bow: switch into Castle age.
6. Madrasah: not only Monks return 33 gold when killed, but Relics generate +50% gold.
Almost same reason with Orthodoxy. More gold give the player more chance to use Camel or Mameluke instead of archers only.
7. Nomad: increase the cost to 450 wood, but make the population cap at the maximum like Huns.
Become true nomads. Nobody spend resources to research this for its original effect instead of training Mangudai.
8. Atheism: cost down to 200 food and 200 gold.
If you have a little extra resources, it is easier to give it a try now.
9. Great Wall: not only Stone Wall and Tower +30% HP, but also makes castle fire scorpion’s arrow as first arrow (or 2~3 extra arrows like Yasama).
The original effect is weaken and more expensive than the tech at University. Chinese has Fortified Wall/Bracer/Architecture/Keep/Arrowslits/Heated Shot/Bombard Tower, full tower and wall, it is pretty enough, making Great Wall unneeded, so it need become more attractive.
10. Paper Money: the player and their allies not only instantly receive 500 gold but also receive 1 gold per 2 or 3 seconds automatically.
Even if it became cheaper than before, it still seems to be research rarely.
11. Silk Road: not only halves the cost of Trade Carts and Trade Cogs, but also halves the cost of Market and Feitoria (including its pop cost), and share the effect to allies.
12. Greek Fire: not only increases the range of Fire Ships, but also let Fire Tower and Flamethrower can be available for the player and their allies.
Just for fun. Make it useful in land map.
13. Kataparuto: just change it to “Bushido”: Samurai get +2 pierce armor and +10% moving speed.
This is impossible but what I want.
Japanese never used Trebuchets and Catapult in the history, but they learn Gunpowder from Portuguese and use it in the civil war. So give Japanese Bombard Cannon to re-balance that.
In the History, the duty of Samurai in the battle is charging at the front, but now they are hard to get closer to archer UU.