In current scholarship, 'Celt' primarily refers to 'speakers of Celtic languages' rather than to a single ethnic group. identified by their use of Celtic languages and other cultural similarities. The relation between ethnicity, language and culture in the Celtic world is unclear and debated. The history of pre-Celtic Europe and Celtic origins is debated. The traditional "Celtic from the East" theory, says the proto-Celtic language arose in the late Bronze Age Urnfield culture of central Europe.A newer theory, "Celtic from the West", suggests proto-Celtic arose earlier, was a lingua franca in the Atlantic Bronze Age coastal zone, and spread eastward. Another newer theory, "Celtic from the Centre", suggests proto-Celtic arose between these two zones, in Bronze Age Gaul, then spread in various directions. The Celts were often in conflict with the Romans,By the 1st century AD, most Celtic territories had become part of the Roman Empire. By c. 500, due to Romanisation and the migration of Germanic tribes, Celtic culture had mostly become restricted to Ireland, western and northern Britain, and Brittany. A modern Celtic identity was constructed as part of the Celtic Revival in Britain, Ireland, and other European territories. The values and key features of Celtic culture are: nature worship, art and creativity, courage and resilience.