Common law helped promise a unified legal system and juries controlled and limited the power of the monarchy, but still having citizens participate through juries in parliament.
In 1066, William of Normandy, a French duke, invaded England. He claimed the English throne. This conquest gradually led to the end of feudalism in England. Feudalism was the political and economic system of the Middle Ages. The conquest also set in motion events that led to the development of democracy in England.
An early development in English democracy was a form of trial by jury. It began in the 12th century. Unlike modern juries, these early juries did not decide whether someone was guilty or innocent. Instead, they were asked by a judge to answer questions about the facts of a case.
Another way that democracy developed in England was through common law.
Common law was not like Roman law, which included things the ruler wanted. Common law was made up of customs and principles established over time. It became the basis of the legal systems in many English-speaking countries, including the United States.
For further reading, see Csun.edu's Democracy Develops in England.