Sanarchy,
It's interesting that you can survive without slavery on high levels. Can anybody else back this up?
It seems odd to me that slavery is not deemed to be an efficient use of resources. To instantly turn excess population into units or buildings, especially when you have reached the happiness cap...what could be more efficient than that?
Well, I never did the math about this as well, but it's my intuition that it's not efficient at all and more a method one should only use in an emergency situation.
More population = more tiles to work, which yields immediate hammers + gold. Also less maintenance cost for your army. When a city grows beyond it's limit it's another story but you still got the

penalty which also kills production etc.
[So when a city is at his cap, then build the workers/settlers, this game is not like previous civ's where you had to expand as fast as possible.]
And what do you get in return? A granary? A library? Those buildings only start to pay of in the long run. [Maybe to have the first border-culture-pop, but a monument costs not much.] [A lighthouse would be an exception maybe] And what can you do with a lot of units if your civilization cannot support them properly?
But then again, I saw posts of people who DID do the math about slavery and who where very enthousiastic about it. But like I said, I only used it once (!) I believe when playing the guy with the sacrificial altar [Montezuma?] and I win most my emperor games and about 30% of immortal games.
I don't have a specific strategy, I mostly think in terms of efficiency. So I take a lot of risks, with always just enough units to reach my goals [too much = too expensive.]
3 supercities are better than 30 crappy ones.
10 veteran units are better than 30 unupgraded.
[Only because they're cheaper to maintain.]
etc..
And of course in warfare the AI is indeed immensly stupid. Plan your battle ahead, get an idea of how many units/and what kind the enemy has. Where can he attack? What are possible strategic points to take, can you maybe destroy his copper/iron? The AI will almost always do the same things. Last game [immortal] I declared war on Cyrus who had about 4 times my army [mine was a little more advanced though, big difference: I had 20 fighters, he about 10.] Took one city the first turn, razed it cause it was a bad one, retreated to my own territory. A few turns later he marches almost half his army into my territory [on 1 tile!] where I slaughter it with maybe 1 or 2 units loss [thanks to fighters & artillery]. The rest was easy...
So, at the point where you can build a decent army, you can hurt EVERY AI, no matter how big his army is.
The only big thing [besides being surprise-attacked] to watch out for on the highest levels is falling behind in techs, that's just a downwards spiral. They'll trade amongst each other while you have nothing to offer in return. Always just let the AI rip you of in trades, without tech brokering at least he can't profit anymore from it. [And you get the nice +4 relations boost.]
Civilizations that are at the bottom of the foodchain can get my techs if they got a little gold to spare if they're fighting with a competitor [so they can put up a good fight.]
Ok, don't wan't to make a strategy-guide or anything. Al those things are already discussed on this forum in detail anyway. But, I really don't get this point about slavery being crucial. Another thing I totally disagree with is the need to micromanage everything. I saw stories about someone who calculated about every turn every cities build-efficiency and moved city-workers around accordingly. That's just nonsense as far as I'm concerned, 1 strategic error will hurt you more than an entire game of not micromanaged cities. Of course you have to be sensible about your buildings/improvements, I mean: the choice of wether to build first a library and then a forge. Or first the forge and THEN the library is more important than if you micromanaged the workers so you could build both in 1 turn less.
[If you catch my drift..]
I mean, maybe if you sold a tech one turn earlier you could have gotten twice the amount of gold for it, who knows? Things like that will impact the game...
One more thing, I don't mean in the beginning of the game, then every turn is micromanaged by me as well.
Damn, what a long story...