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
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.