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Serfdom is a labor civic in Civilization IV.
Strategy
Serfdom allows Workers to build improvements faster.
Civilopedia entry
Serfdom is a more "enlightened" form of slavery in that the power of the master is not absolute and the serfs are accorded a certain number of rights. The specific rights and obligations of masters and serfs vary significantly over time and from place to place. In some societies, the master had the right to control every aspect of a serf's life, except that he couldn't kill the serf without just cause. In others, the serfs had the rights to marry and to inherit wealth and to a certain amount of self-government, but they could not leave the land without their lord's permission.
The basic premise of serfdom is that one group of people are superior to another, and that they have the political and god-given right to control the other group's destiny. Thus serfdom only makes sense if one agrees with the group currently in power about who is superior to whom.