Spanish Citadel (Castle):
Patched Warlords made Castles more useful by giving them +1 trade route and -25% damage from bombardment from non-gunpowder units in addition to the +50% vs. non-gunpowder units and +1 culture. Cumulative with the prerequisite Walls, that means +100 defence from non-GP units and -50% from non-GP bombardment.
The Citadel gives an additional 25% bombardment protection for a total of -75%
and gives 5XP to seige units produced in the city. From the +3XP barracks and the Citadel that is 8XP already, but Thoecracy and Vassalge give 12XP from the start enough for 3 promotions. The bonus is applied to Trebuchets, Catapults and if you can avoid the obsoleting tech, Economics (which restores the lost trade route, 1st to discover gets a GM) you can have Cannons, Artillery and even machine guns with this bonus as well.
Possible Synergies:
The strength of the UB lies in how one promotes their seige weapons most effectively. A warmonger or early-war-to-to-claim-land strategy uses it to it's full advantage, but just protecting your cities becomes much easier now too. Let's take a look at the Seige Units and remember they cannot receive defensive bonuses:
Code:
[U]Unit Cost Strength Movement Bonuses [/U]
Catapult 40 5 1 25% Withdrawal, Causes Collateral Damage
Bombard City Defences (-15%/turn)
Trebuchet 60 4 1 25% Withdrawal, Causes Collateral Damage
+100% vs. City, Bombard City Defences (-25%/turn)
Cannon 100 12 1 25% Withdrawal, Causes Collateral Damage
Bombard City Defences (-20%/turn)
Machine Gun 120 18 1 1st Strike, Only Defends, +50% vs. GP Units
Artillery 100 18 1 25% Withdrawal, Causes Collateral Damage
+50% vs. Seige, Bombard City Defences (-25%/turn)
All these units receive Barrage, Drill, Shock, Ambush, and City Raider (except Machine Gun who only defends) as well as the Combat and Medic promotions. Any Grenadiers promoted to Machine Gun will not gain any XPs if ungraded in a Citadel, but any you build there will. None of these units can receive City Garrison, Cover, Pinch, Formation, Charge or (Guerilla or Woodsman), but it is possible to have your Grenadier or Melee Units promoted into Grenadiers and give
them any of those promotions and then upgrade them into Machine Gunners. You could for example create G2 or G3 Machine Gunners with Ambush and CG promotions to defend a Hill city from Tanks or you could have Woodsman Machine Gunners with Pinch and Medic/Charge defending your SoD on route through a forest to attack an enemy city.
Options for your other seige units are even more numerours. Except for the Machine Gun, all the seige units have a 25% withdrawal chance, if you add a GG to it, you can it 55% as well as making it a nice Seige Weapon from other promotions. Remember that upgrades are
free when attached to a Warlord, so you could have some expert Machine Gun defenders or great Cannons/Artillery once the competition gets a little tougher and the AI has more advanced units. One other thing that really stood out to me in trying the UB out, was that Seige Units are immune to collateral damage, so having plenty of them in your stack means you will have some to attack with while other units might need to heal. You can give them medic promotions too remember.
As for synergy with the Conquistador, it is a knight with +50% vs. melee units. Many different civs have melee units for their UU, which comes in handy. With a few correctly promoted Conquistadors and Seige weapons the two should complement each other nicely. Don't forget to balance out your forces, using more than just these two.
Possible Drawbacks?
This UB doesn't really do anything for the Expansive trait or the Spanish starting tech of fishing and mysticism, however being religious does offer advantages to being able to switch to new civics immediately and not waiting until others are discovered. A switch to Theocracy before you wait to get Vassalage could mean the difference between producing units with an extra +2 XPs and for the Seige units with the Citadel, that is enough to put them over the top for a third promotion.
Summary:
If you want to avoid Economics, the obsoletion tech, you only really miss out on a Great Merchant.

You could conceivably hold out for some super artillery and machine gunners if you wanted. This UB allows for some intersting warface capabilities. Although it doesn't immediately lend it self to anything other than conquest/domination, the extra territory and cities gained or even the ability to better protect one's own cities and territory is nothing to take lightly.