One additional note that makes slavery a very powerful civic is the concept of production overflow. When you whip away population you always whip away as many as you need to finish a build, with whatever amount is excess becoming overflow for the next build. So, say you want build an army of 35
Axes, and you've got 4
invested into an Axe. One pop would only put it at 34/35
, which isn't enough to finish the build (even though it technically is given the city tile alone always produces 1
, but whipping only looks at the current state of the production bar and ignores everything else). So you need to two-pop whip the axe, putting the build at 64/35
. Let's say that city produces 2
from tile yields, so next turn you've got an Axeman and 31
overflow. Sink the overflow and the city's 2
into a new axe, and you've got a new Axeman at 33/35
the turn after you just pumped one out of that city, and a second Axeman the turn after.
Remember that, with a Granary, a wet corn has the effective production yield of about three plains hill mines, and you can probably see how slavery lets you pump out an army in record time.
Of course that isn't the only application of production overflow. Another option is to put that production overflow into a wonder in order to generate a fair bit of failgold, or just give it a quick injection of hammers. Directly whipping wonders is penalized, you don't get as many hammers per pop as you do whipping a unit or regular building, but whipping a unit or regular building and putting overflow into a wonder is perfectly fair game.