According to Roman tradition, in 753 BC when Romulus founded the city of Rome and instituted the monarchy, he also created the office of custos urbis (guardian of the city) to serve as the king’s chief lieutenant. Appointed by the king to serve for life, the custos urbis served concurrently as the Princeps Senatus. As the second highest office of state, the custos urbis was the king’s personal representative. In the absence of the king from the city, the custos urbis exercised all of his powers, which included the powers of convoking the Senate, the popular assemblies and the exercise of force in the event of an emergency. However, the imperium he possessed was only valid within the walls of Rome.
Under the kings, only three men held the position. The first king Romulus appointed Denter Romulius to serve as the first custos urbis, the third king Tullus Hostilius appointed Numa Marcius, and the seventh king Tarquinius Superbus appointed Spurius Lucretius.