Slavery allows its adopter to finish production by sacrificing a city's population.
Slavery is a system whereby some people are judged far inferior to others and are treated as objects or possessions, with no more rights than those afforded to chairs or cattle. Slavery has existed as long as human history, from earliest primitive times to the enlightened days of Greece and Rome and right up until today. Sometimes convicted criminals were made into slaves. Sometimes captured enemies of the state were enslaved. Sometimes people of a specific race or coloration were enslaved. In some civilizations a person could purchase their freedom or were allotted it after being enslaved for a specific period of time.
The advantage of slavery is that it allows people to be forced to do jobs that they would otherwise refuse to do without adequate compensation. Cheap labor results in concentration of wealth and thus the creation of great buildings and temples or the maintenance of large professional armies. Aside from its basic moral repugnance, the disadvantages of slavery are numerous; it corrupts both slave and master, it badly underutilizes the intelligence and creativity of the subject people, and it almost inevitably leads to revolts and internal strife.