What came to the rescue was a Feudalism trade. Vassalage's free units are highly underrated, saving me half my army cost here. Also to the rescue were a few marketplaces, mostly just to hire the merchants.
So I sent my lead offense stack to Teotihuacan, and assembled a second stack to start from Tlaxcala. Trebuchets ripped apart defending archers and jaguars, and elephants cleaned up. It was odd running an invasion without any proper normal units to garrison the cities (archers are superseded but I don't have maces or longbows yet.) But I made do with pikemen and crossbows. The war lasted to 880 AD, just 12 turns to eliminate the Aztecs.
The war satisfyingly crashed my economy, sinking as low as -15 gold at 0% science and surviving on conquer cash. Unit and supply and inflation costed over 40 gold per turn! So how do we dig out of this? Cottages are not the answer. Alien cottage production can be converted back into economy via Wealth, but then I'm not building more military.
What came to the rescue was a Feudalism trade. Vassalage's free units are highly underrated, saving me half my army cost here. Also to the rescue were a few marketplaces, mostly just to hire the merchants.
Yes, both AIs beyond Montezuma got Feudalism. Was this a disaster, now having to beat longbows with trebuchets? Not at all. I _wanted_ that to happen. I don't really want to romp with trebs over archers. I want to see what Spain's CR3 trebs can do when picking on somebody their own size.
Madrid was now quite a strong Great Person producer, with Nat Epic and Great Library. GP #3 came only ten turns after the previous. He was a Scientist and definitely went for a Golden Age. That Golden Age came earlier than in my other game, and it's exactly what I wanted to see from the mod. It added a good 12 hammers per city (15 after forge), spiking populations up to 18-20 each, and the commerce produced lots more military.
The ensuing Golden Age itself cashed out GP #4 from my Oracle city, an Engineer at low odds. What could he do? Maybe Notre Dame, or...
Lightbulb Guilds. Never thought of trying to do that, but it's brilliant here. Knights add some good punch to my offense, a couple Grocers further keep me from bankruptcy, and the workshop boost is very welcome. Plus it even opens up Banking for more money. And Mercantilism is all upside once I declare war on Hannibal which blocks trade routes with Germany.
GP #5 from the now-mixed pool was a Scientist. Bulbing Philosophy is no help. I held on to him for a Golden Age that never came. GP #6 came as an Artist, and was saved for a Great Work towards domination along with the Music Artist.
With the Aztecs gone, Carthage is now next on the list.
Wow! Hannibal's got quite a force ready on defense. Looks like I'll get the fight I wanted! I think he was planning an attack on Montezuma - didn't check for WHEEOH before killing Monty. Well, my force there should be up to the task. Yes, I'm fighting with every medieval unit _except_ macemen. Haven't had the economy to go for Civil Service, but I was able to bulb three other unit techs in Machinery-Engineering-Guilds. Hannibal still lacks Metal Casting, so no danger of pikemen stopping my mounted stack or castles resisting my siege.
Well, I didn't exactly get the fight I wanted. Instead of shredding with trebuchets inside a city, Hannibal decided to suicide his stack against mine on the forest tile. I'll take the result though. After that, I blasted through Carthaginian cities as fast as the trebuchets could reach them.
And now my economy went seriously apocalyptically negative. Hannibal built the Statue of Zeus after I conquered two cities. 30% culture for me, sending my income to -80. I actually had to rely on PILLAGING for cash. I've pillaged maybe a dozen tiles in my entire Civ 4 career. Never worth it since you'd rather conquer intact territory to keep. But here it was necessary.
The Carthage war lasted to 1120 AD, so 16 turns to kill him. He had a couple island cities left, so I just took his vassalization.
An amusing detail that I didn't realize in the space game: While a conquered city is in resistance, you can chop forests into its food box to avoid starvation when it comes out. 
In the tech department, I did limp to Music and score the Great Artist, saved for a culture bomb. And a bit later I also made it to Banking on conquer cash, going to Mercantilism for a bit of specialist help. And I got Germany to flip me Civil Service for Music + Drama, finally enabling macemen. (Avoided trading him both Guilds and Engineering.)
Conquest of Germany went pretty much as over Hannibal, just smashing with trebs, cleaning up with elephants and knights, and a couple crossbows and pikes tagging along for stack defense.
City Raider 3 trebuchets are indeed overpowered. They conquer cities with almost no losses. After bombardment, a treb attacks at 4 + 175% = 11 strength. That's about equal to longbows in most situations (6 +25% innate +25% fortify + a CG promo or two). But the trebuchet is so good at avoiding losses because it only has to win 75% of the battle then withdraws. It's like a tennis match where one player needs only three points for every game instead of four. And after one treb has withdrawn, the next treb is attacking a weakened collateralized unit, and it snowballs from there. (I realize all of this is old news and has nothing to do with the mod. But it's the first time I've done this myself.)
And this ending played out just as I predicted in the game setup. City population and production are both higher than usual with the Isabalien mod, so we get lots of units. I had my Heroic Epic city at one trebuchet per turn for the entire game. And for players that wanted to experience the wacky mod and then get it over with, the smashy victory worked out just fine, either with trebs or conquistadors or cannons. But it would require economic management along the way - land tiles just don't produce food in quantities anywhere near that of commerce.
I didn't bother taking pictures of the German conquering. There was just one tactical sequence worthy of note.
On the first turn of the war, I'd captured Bremen. I had a leftover scout in the area, so sent him to go see if Essen might be lightly defended. It was, so I jumped on the chance to grab the city quickly with knights rather than waiting for siege to trudge there. I sent overkill of 7 knights just in case the AI brought any defenders in. Also, there happened to be a worker on the hill that I grabbed.
And it seemed the right time to fire a Great Work to claim a huge chunk of tiles towards domination. The Great Work got control of the circled tile. I sent a spy towards Frankfurt to check on its defenses. Lightly defended again. So I moved the captured worker to the desert tile to start roading it. Don't care if it gets captured.
To my aghast surprise, the worker FINISHED said road. The tile had been partially roaded already. So my knight stack could also reach Frankfurt all still on the SAME TURN, and razed it. There's some multiplayer style conquering for ya.
Three turns later, I reached Munich and popped another Great Work to end it. Domination Victory in 1190 AD.
So how did those City Raider 3 trebuchets do?
I lost well under one treb per conquered city. The final score (minus workers) was 30 units lost, 165 units killed. The entire power graph shows almost no visible losses.
Turn the page for some more thoughts.