The most important principle of Roman law was that it should be written and transparent. That is, everyone should know what the law was and the law should not simply change based on the whim of a ruler or judge. This idea of the rule of law was the basis of all Roman law.
Another major principle of Roman law was that it should apply to all aspects of life. There would be laws about criminal acts and laws about contracts and laws about family life.
A third principle was that both government and tradition should be sources of law. There were laws (jus scriptum) that were written and tended to come from governmental sources. But there were also unwritten laws that had come from tradition and precedent. These were called "jus non scriptum" and were similar to the later English tradition of the common law.
1) All citizens had the right to equal treatment under law.
2) A person was considered innocent until proven guilty.
3) The burden of proof rested with the accuser rather than the accused.
4) Any law that seemed unreasonable or grossly unfair could be set aside.