Following Tullus’ mysterious death, the Romans elected a peaceful and religious king in his place. The king they elected was Numa’s grandson, Ancus Marcius. Much like his grandfather, Ancus did little to expand the borders of Rome and only fought war when his territories needed defending. He also built Rome's first prison on the Capitoline Hill.

During his reign, Janiculum Hill on the western bank was fortified to further protect Rome, and the first bridge across the Tiber River was built by Ancus. He would also found Rome’s port of Ostia on the Tyrrhenian Sea and establish Rome’s first salt works. During his reign, Rome's size increased as Ancus used diplomacy to peacefully join some of the smaller surrounding cities into alliance with Rome. Through this method, he completed the conquest of the Latins and relocated them to the Aventine Hill, thus forming the plebeian class of Romans.

He would die a natural death, like his grandfather before him, after 25 years as king, and would be remembered as one of Rome’s greatest pontiffs.