Neolithic colonists integrated with the indigenous people, gradually changing their lifestyles from a nomadic life of hunting and gathering, to become settled farmers about 6,000 BP – the Neolithic Revolution. They cleared the forests to establish pasture and to cultivate the land, developed new technologies such as ceramics and textile production, and built cromlechs such as Pentre Ifan, Bryn Celli Ddu, and Parc Cwm long cairn between about 5,800 BP and 5,500 BP. Over the following centuries they assimilated immigrants and adopted ideas from Bronze Age and Iron Age Celtic cultures. Some historians, such as John T. Koch, consider Wales in the Late Bronze Age as part of a maritime trading-networked culture that included other Celtic nations. This “Atlantic-Celtic” view is opposed by others who hold that the Celtic languages derive their origins from the more easterly Hallstatt culture. By the time of the Roman invasion of Britain the area of modern Wales had been divided among the tribes of the Deceangli, Ordovices, Cornovii, Demetae and Silures for centuries.
The Roman conquest of Wales began in AD 48 and took 30 years to complete; the occupation lasted over 300 years. The campaigns of conquest were opposed by two native tribes: the Silures and the Ordovices. Roman rule in Wales was a military occupation, save for the southern coastal region of south Wales, where there is a legacy of Romanisation. The only town in Wales founded by the Romans, Caerwent, is in south east Wales. Both Caerwent and Carmarthen, also in southern Wales, became Roman civitates. Wales had a rich mineral wealth. The Romans used their engineering technology to extract large amounts of gold, copper and lead, as well as lesser amounts of zinc and silver. No significant industries were located in Wales in this time; this was largely a matter of circumstance as Wales had none of the necessary materials in suitable combination, and the forested, mountainous countryside was not amenable to industrialization. Latin became the official language of Wales, though the people continued to speak in Brythonic. While Romanization was far from complete, the upper classes came to consider themselves Roman, particularly after the ruling of 212 that granted Roman citizenship to all free men throughout the Empire. Further Roman influence came through the spread of Christianity, which gained many followers when Christians were allowed to worship freely; state persecution ceased in the 4th century, as a result of Constantine I issuing an edict of toleration in 313.
Early historians, including the 6th-century cleric Gildas, have noted 383 as a significant point in Welsh history. In that year, the Roman general Magnus Maximus, or Macsen Wledig, stripped Britain of troops to launch a successful bid for imperial power, continuing to rule Britain from Gaul as emperor, and transferring power to local leaders. The earliest Welsh genealogies cite Maximus as the founder of several royal dynasties, and as the father of the Welsh Nation. He is given as the ancestor of a Welsh king on the Pillar of Eliseg, erected nearly 500 years after he left Britain, and he figures in lists of the Fifteen Tribes of Wales.