tmit - your analysis is pretty ok, but you forget a key element: till rene, the cap on units is based on cash, not production. Obviously you can exploit the faster unit production, but you'll end up being broke; on high levels at least. Plus the ai checks you on military and you'll fail miserably far more turns. Even with caesar and already with col, fighting izzy(which was researching dr instead of feudalism), me whipping ch when I was taking a city, still I was broke.
pret. production was fine, research wasn't. Obviously I would've recovered, since I had 3 times the land of the other ai, but till recovering, 2 ais from the other continent declaring took me out. Praets are good, but not against cuirassiers...
leaving aside that in my limited exp. on normal, the defenses were clearly lower(also clearly not 33% lower, probably something around 25% lower). Though ok, when you start attacking, probably it's already the end - the problem is usually till you start your attack.
where I agree marathon is easy - if you have no imagination, play rome, build only praets, play pangeea, probably you can finish them game by building only praets... but normally you'll do that once-twice till you get bored. Probably with WE/cata spam too since even if they get to pikes, they don't spam the them.
I was going with the 33% for comparing marathon to quick, which is kind of heresy because quick is awful

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I'd argue the cap on units expires long before the renaissance, and more in the period of currency/CoL, with a few cottage cities grown and markets, rest hammers. Granted, I've never attempted marathon deity, but I have played/beaten every other level marathon multiple times (except the really low ones).
I've actually played standard continents on EPIC and cleared my entire half of the world with just keshiks on immortal - tech extortion, tactics, and basic courthouse/empire management come into play.
My experience with marathon diplo mostly comes from mad's RPC's and HoF, but I don't find it TOO different from normal. You're going to fail war checks above monarch no matter what you do and on every speed. With reasonably high odds for declarations at cautious, odds are you'll get stomped even on normal speeds if you piss off the AIs/ignore diplo (though it's easier to defend). However, the AI is pretty well-manipulated. What I find marathon really forces is taking sides. On large maps the diplo hurts like hell, but on standard maps you can easily get 2-4 AIs to the point that they will never, ever declare on you. Outside of the aforementioned pangaea, this leaves long stretches of the game where you're immune to war (knowing the war, vassal, and AI mechanics and abusing them to hell really helps, but this is probably my single strongest play ability - much better than my micro and probably even better than my warmongering).
1. Everyone building units faster is not giving human an advantage. It speeds up everyone. "Faster" means building a unit takes less time. For example replenish losses or war preparations.
The argument is that the AI will build units at a fixed rate regardless of their flagrantly reduced cost. They ignore it entirely. Unless the human ignores it entirely too, it's an advantage for the human.
2. Having AI build less units is not giving human an advantage. It makes the game more difficult for a builder human, since the AI will build more buildings and thus have better economy.
A builder human IS arguably disadvantaged on marathon. However, no player who wins consistently on high levels is going to be called a "builder" or a "warmonger". Inability or lack of desire to take care of some aspect of civ IV is a hole in one's play. So, if one is bad at military prep/usage, marathon will expose this flaw more so than normal. Make no mistake though, it's a flaw. Show me one top notch player that ignores either his tech rate OR his military completely on a consistent basis.
3. Not sure what it is supposed to mean. Are you saying that it is impossible to build a bigger stack than AI's on lets say Epic? And possible on Marathon?
Of course I'm not saying that

. You can build a bigger stack than the AI on quick. What I AM saying is that it's easier on marathon specifically, because even as units become cheaper, the AI continues building them at the same rate, not taking advantage of this bonus. We're basically comparing a 0% human and say 20% AI bonus on other speeds with a 50% human and 70% (or whatever) AI bonus on marathon, but the AI will still train the same # of units. It doesn't adjust, just as it doesn't on water maps or other maps and thus does more poorly.
4.As for the biggest gripe, it works both ways. It is more difficult to repel invasions and to defend against backstabbers. Units take forever to build. And no hope of invading army becoming obsolete too. Wars are more interesting on Marathon, mainly becausing they are decided by strength and tactic, not whose stuff obsoletes first. Or making a right peace treaty at the righ time and in 10 turns his army is junk.
I agree with this. I shouldn't have numbered it. It's just something that *really* irks me about quick, which is the speed I'd play otherwise. But that isn't really relevant to a discussion on marathon haha. Repelling invasions is easier if you are prepared on marathon, but if you AREN'T you're toast.
But all in all the argument is actually a simple one:
The AI was created and balanced based on a standard map and at normal speed. ANY variation to map, speed, and so forth hurts the AI because its programming is still fixed. It won't adjust, even doing ridiculous things like building a large military in always peace or having 3x your power but landing with 1/16th of its power for an intercontinental invasion. Marathon is just another example of this.
Of course, marathon barbs can still rape you, and bad starts with hard diplo or boxed in without resources are probably harder. But on average, the holes in AI adaptability make it weaker on marathon.