Answer:
Criminal acts:
Those who have broken the law, but are not worthy of death, usually served prison time or menial labor until their sentence is served.
To pay off debts:
Those who have large amounts of debt but had no money to pay generally were able to pay off their debt through working for their debtor.
Prisoner of war:
Prisoners were a large source of potential slaves who can be used to help construct infrastructure within a nation. Examples can be found in Rome subduing many of the Barbaric tribes in the North.
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