During the Iron Age, Celtic culture, deriving from the Hallstatt and La Tène cultures, arrived from Central Europe. Brythonic was the spoken language during this time. Society was tribal; according to Ptolemy's Geographia there were around 20 tribes in the area. Earlier divisions are unknown because the Britons were not literate. Like other regions on the edge of the Empire, Britain had long enjoyed trading links with the Romans. Julius Caesar of the Roman Republic attempted to invade twice in 55 BC; although largely unsuccessful, he managed to set up a client king from the Trinovantes. The Romans invaded Britain in 43 AD during the reign of Emperor Claudius, subsequently conquering much of Britain, and the area was incorporated into the Roman Empire as Britannia province. The best-known of the native tribes who attempted to resist were the Catuvellauni led by Caratacus. Later, an uprising led by Boudica, Queen of the Iceni, ended with Boudica's suicide following her defeat at the Battle of Watling Street. This era saw a Greco-Roman culture prevail with the introduction of Roman law, Roman architecture, aqueducts, sewers, many agricultural items and silk. In the 3rd century, Emperor Septimius Severus died at Eboracum (now York), where Constantine was subsequently proclaimed emperor. There is debate about when Christianity was first introduced; it was no later than the 4th century, probably much earlier. According to Bede, missionaries were sent from Rome by Eleutherius at the request of the chieftain Lucius of Britain in 180 AD, to settle differences as to Eastern and Western ceremonials, which were disturbing the church. There are traditions linked to Glastonbury claiming an introduction through Joseph of Arimathea, while others claim through Lucius of Britain. By 410, during the Decline of the Roman Empire, Britain was left exposed by the end of Roman rule in Britain and the withdrawal of Roman army units, to defend the frontiers in continental Europe and partake in civil wars. Celtic Christian monastic and missionary movements flourished: Patrick (5th-century Ireland) and in the 6th century Brendan (Clonfert), Comgall (Bangor), David (Wales), Aiden (Lindisfarne) and Columba (Iona). This period of Christianity was influenced by ancient Celtic culture in its sensibilities, polity, practices and theology. Local "congregations" were centred in the monastic community and monastic leaders were more like chieftains, as peers, rather than in the more hierarchical system of the Roman-dominated church.