The Twelve Caesars', is a set of twelve biographies of Julius Caesar and the first 11 emperors of the Roman Empire written by Gaius Suetonius Tranquillus. The work, written in AD 121 during the reign of the emperor Hadrian, was the most popular work of Suetonius, at that time Hadrian's personal secretary, and is the largest among his surviving writings. 'The Twelve Caesars' is considered very significant in antiquity and remains a primary source on Roman history. Augustus was the founder of the Roman Empire and its first Emperor, ruling from 27 BC until his death in 14 AD. His reign initiated an era of relative peace known as the Pax Romana (The Roman Peace). Despite continuous wars of imperial expansion on the Empire's frontiers and one year-long civil war over the imperial succession, the Roman world was largely free from large-scale conflict for more than two centuries. Augustus dramatically enlarged the Empire, expanded possessions in Africa, expanded into Germania, and completed the conquest of Hispania. Beyond the frontiers, he secured the Empire with a buffer region of client states, and made peace with the Parthian Empire through diplomacy. He reformed the Roman system of taxation, developed networks of roads with an official courier system, established a standing army, established the Praetorian Guard, created official police and fire-fighting services for Rome, and rebuilt much of the city during his reign.