The document that established the fundamental rights of citizens in Ancient Rome is the Twelve Tables. Created around 450 BCE, the Twelve Tables were a set of laws inscribed on bronze tablets that outlined the rights and responsibilities of Roman citizens. This legal code represented a crucial step towards the concept of written law, ensuring public access to legal rights and protections. It addressed various aspects of everyday life, including property rights, family law, and legal procedures. The Twelve Tables set a precedent for later Roman law and continue to influence legal systems in the modern world.