said to be granted to the Romans by their supreme deity Jupiter. This claim of universal dominion was renewed and perpetuated when the Empire came under Christian rule in the 4th century. In addition to annexing large regions in their quest for empire-building, the Romans were also very large sculptors of their environment who directly altered their geography. For instance, entire forests were cut down to provide enough wood resources for an expanding empire. In reality, Roman expansion was mostly accomplished under the Republic, though parts of northern Europe were conquered in the 1st century AD, when Roman control in Europe, Africa, and Asia was strengthened. During the reign of Augustus, a "global map of the known world" was displayed for the first time in public at Rome, coinciding with the composition of the most comprehensive work on political geography that survives from antiquity, the Geography of the Pontic Greek writer Strabo. When Augustus died, the commemorative account of his achievements (Res Gestae) prominently featured the geographical cataloguing of peoples and places within the Empire. Geography, the census, and the meticulous keeping of written records were central concerns of Roman Imperial administration. The Empire reached its largest expanse under Trajan (), encompassing an area of 5 million square kilometres. The traditional population estimate of inhabitants accounted for between one-sixth and one-fourth of the world's total population and made it the largest population of any unified political entity in the West until the mid-19th century. Recent demographic studies have argued for a population peak ranging from to more than . Each of the three largest cities in the Empire – Rome, Alexandria, and Antioch – was almost twice the size of any European city at the beginning of the 17th century. As the historian Christopher Kelly has described it: Trajan's successor Hadrian adopted a policy of maintaining rather than expanding the empire. Borders (fines) were marked, and the frontiers (limites) patrolled. The most heavily fortified borders were the most unstable. Hadrian's Wall, which separated the Roman world from what was perceived as an ever-present barbarian threat, is the primary surviving monument of this effort. Languages The language of the Romans was Latin, which Virgil emphasized as a source of Roman unity and tradition. Until the time of Alexander Severus (), the birth certificates and wills of Roman citizens had to be written in Latin. Latin was the language of the law