How did they recruit?

During the peak of the Roman Empire, the legions of Rome were recruited (voluntarily) from the citizen class and mainly from rural areas, due to their more hardy nature.

A letter of recommendation was needed from a noteworthy or influential figure, and bad references could lead to a citizen only able to join the (shock, horror) less prestigious navy. Other than being in good physical condition, one also had to be a man, over 20 years of age and expected to serve for a minimum of 25 years.