The Constitution of the Roman Kingdom was an unwritten set of guidelines and principles originating mainly through precedent. During the years of the Roman Kingdom, the constitutional arrangement was centered on the king, who had the power to appoint assistants, and delegate to them their specific powers. The Roman Senate, which was dominated by the aristocracy, served as the advisory council to the king. Often, the king asked the Senate to vote on various matters, but he was free to ignore any advice they gave him. The king could also request a vote on various matters by the popular assembly (the "Curiate Assembly"), which he was also free to ignore. The popular assembly functioned as a vehicle through which the People of Rome could express their opinions. In it, the people were organized according to their respective curiae. However, the popular assembly did have other functions. For example, it was a forum used by citizens to hear announcements. It could also serve as a trial court for both civil and criminal matters.
The period of the kingdom can be divided into two epochs based on the legends. While the specific legends were probably not true, they were likely based on historical fact. It is likely that, before the founding of the republic, Rome had actually been ruled by a succession of kings. The first legendary epoch spans the reigns of the first four legendary kings. During this time, the political foundations of the city were laid, the city was organized into "curiae", the religious institutions were established, and the Senate and the assemblies evolved into formal institutions. The city fought several wars of conquest, the port of Ostia was founded, and the Tiber River was bridged. The early Romans were divided into three ethnic groups: the Ramnes (Latins), Tities (Sabines), and Luceres (Etruscans). The original "patrician" families belonged to these ethnic groups. In an attempt to add a level of organization to the city, these patrician families were divided into units called "curiae". The vehicle through which the early Romans expressed their democratic impulses was known as a "committee" (comitia or "assembly"). The two principal assemblies that formed were known as the "Curiate Assembly" and the "Calate Assembly". The two assemblies were designed to mirror the ethnic divisions of the city and, as such, the assemblies were organized according to curia. The vehicle through which the early Romans expressed their aristocratic impulses was a council of town elders, which became the Roman Senate. The elders of this council were known as patres ("fathers"), and thus are known to history as the first Roman senators. The populus ("people") and the elders eventually recognized the need for a single political leader, and thus elected the rex (king). The populus elected the rex, and the elders advised the rex.