I took a look at comparing
Slavery to
Caste and I think we can do some adjusting here.
First, reducing the
Slave Market:
- Remove the production and gold bonuses.
- Add flat +3 gold/turn. I think +3 is okay, but I would rather have gold here rather than commerce.
- Possibly add an additional -1 health.
- That leaves 1 free Slave, +3 gold/turn, and -1 happy.
Second, boost
Caste a little:
- Remove the largest city unhappiness. I think the GPP penalty is the more appropriate drawback, and Caste doesn't need two different negatives.
Third, for Revolutions only:
- Slavery should not grant a stability bonus for switching to. If anything, the sign should be reversed and should increase instability.
How does this look so far?
I've been playing around with this a bit. I think one change that might help balance these civics a bit would be to increase the Upkeep for Slavery. The Slavery bonuses are just so powerful, with or without the Slave Market. However the civic feels flavorless without them.
I'm not sure about that change to the Slave Market. It's pretty clear that you want the Slave Market and slavery in general to be early-game only, but if anything giving it flat +3

is actually too strong early on. A city would need to be generating 30 base

/turn before the old slave market would outpace the suggested new one, and thats not even accounting for the fact that all other early +% economy buildings are +%

only, like Bazaar from Trade. No early game civ requires that much

.
Slave Market is already one of the strongest buildings to build first in any new city, considering that the free specialist can double a young city's

output, and the suggested change would only enhance that. It would essentially make all new cities immediately profitable, allowing slaver civs to expand like mad with essentially no economic penalty while still running a massive

slider.
I think we need a different approach to the Slave Market than that. I do think I agree with you though that the +15%

to Slave Market could be removed. That bonus doesnt really make sense to that building, unless I'm missing something, and it is a very large part of what makes Slavery flat out superior to Caste at the moment. Perhaps the +%

on slave market could be replaced with a small +% to the production of workers?
I also very strongly agree with you that we need to be looking at a direct comparison between Slavery and Caste, considering their relative position as competing Civics via their position in the Tech tree.
Personally, I'm not really sure that any of the bonuses or penalties for the Caste civic fit at all, but I dont consider myself particularly educated about any of the social systems that westerners retroactively called caste either. Modern scholars seem a bit divided on what Caste systems in fact meant in different places at different times in history. Perhaps this is because the word Caste is a touchy political issue contemporarily in the Indosphere, I dont know. Civ4 is a game though, and has traditionally been perfectly willing to ignore historical realities in favor of dealing in essentially History Memes (ie 3-5 sentences that could be used to explain a broad topic to a 3rd-grader), since such memes are easier to translate into a game. I would love discussion on the subject, what should/does this Civic provide in terms of game-sense, what historically should this civic reflect?