Roman law was civil because, with the establishment of the Twelve Tables in 5 B.C.E., every citizen received the same treatment for certain crimes. The Twelve Tables are considered one of the foundations for civil law systems.
A civil system is a system that relies on established codes of law in order to determine how to execute legal justice. The term "civil" has its origins in the Latin word for citizens. The Twelve Tables were established when the large working class of Rome demanded that they be granted certain legal rights in the name of fairness. Following a threat to secede, the Twelve Tables were created.
Some elements of the Twelve Tables are still found in American law as of 2014. For instance, the idea of having 30 days to pay off debt appeared in the Twelve Tables. The Twelve Tables also stated that both parties must be present in court. If one fails to report, a judge may, by default, rule for the party that shows. Another similarity between Roman law and U.S. law is the certain crimes carry mandatory punishments based on the severity of the crime.
The United States is not completely a civil law system. There are elements of common law in the U.S. judicial system. Common law is a system of law in which a precedent determines the outcome of future cases of a similar nature.