There were seven traditional Kings of Rome before the establishment of the Roman Republic. They were, according to the writings of Livy:
The earliest kings and dates may well be mythical.
Rome was, according to tradition, founded in 753 BC by Romulus and Remus, twin sons of the mortal woman Rhea Silvia and the god Mars, and raised by a female wolf. They were also descendants of Aeneas and the Trojan refugees whose story Virgil later told in his epic poem the Aeneid. Romulus killed Remus, and became the first king of Rome (see founding of Rome). Most of the succeeding six kings had Etruscan names, suggesting that members of the mature Etruscan civilization to the north of Rome dominated the city.
The last king was thrown out by the citizens and replaced by a republican government. The expulsion of the king and the founding of the Republic in 509 BC is sometimes presented as the breaking away of a Latin-speaking population from the control of an Etruscan ruling family.
Unlike many other Italian city-states of that time, the Roman monarchy was not totally based on inheritance. Once a king died, the city entered into a period of interregium. The city was ruled by an interrex who had the power to nominate the next King. The interrex was nominated by the Senate, but served for an indefinite period of time. Once the interrex found a candidate he then submited him to the Comita Curiata, an assembly of the people. If he was approved by that body, the Senate then ratified the vote. In theory the people got to elect their leader, however the Senate had most of the control over the process. Traditionally the interregia were less than one year; however, then the average reign of a king would have been over 34 years. If longer interegia are taken into account, it is possible that the seven kings may have had less time on the throne then previously thought.
The system for choosing the king broke down after the murder of Tarquin I. Tanaquil, the wife of Tarquin managed to place Servius Tullius in power as the king, although he had not been elected to become king. During his reign, Tullius held a referendum on his monarchy, which overwhelmingly approved of him. Servius Tullius was overthrown by Tarquin II (the proud) the (grand)son of the first Tarquin. Tarquin II would be the last Roman monarch.
During this period, Rome's growth was made possible after the drainage of the swamps that are the natural feature of the site. As the site was in the center of the early kingdom, it was the obvious spot to improve cohesion by constructing a central market (a forum). This resulted in combining the Romans into one people. Napoleon Bonaparte restored the drainage systems, making excavation of the Forum Romanum possible.