Before the Roman Empire's conquest, Britain and Ireland were populated by Celtic migrants from the European continent, but ones who left no recorded history of law. Near the end of the Roman Republic in 55 and 54 BC, the former Consul and legion commander Julius Caesar invaded Britain during the broader Gallic Wars. This did not establish permanent occupation, as Caesar returned to Rome, became dictator and was assassinated. The Republic was transformed into an Empire, when Caesar’s heir Augustus took power in 27 BC. In the reign of Augustus’ wife Livia’s grandson, Claudius, Britain was conquered from 43 AD. Under Rome's uncodified constitution, Roman Britain was administered by a governor, usually member of the Senate but appointed by the Emperor for their military record. Londinium was a provincial capital of 60,000 people, and Britain a cosmopolitan society of around 3 million people. Roman law was based upon a slave economy, and highly militarised. Hadrian constructed a wall from 122 as part of the Empire's limits, but this was soon moved north by Antoninus Pius from 142. Constantine the Great was stationed in York in 306 when he left to claim his title to be Emperor. Constantine marched on Rome in 312, and issued an Edict of Milan in 313. This triggered a series of events where the Church assumed more and more power over the law. But under constant assault, the Empire began to collapse and Britain was abandoned in 407. Neither the Theodosian Code issued in 438, nor the great "Corpus Juris Civilis" of Justinian I in 534 entered the laws of Britain.