 Gallo-Roman religion formed when the Roman Empire invaded and occupied the Brythonic peoples.Elements of the native Brythonic Celtic religion such as the druids, the Celtic priestly caste who were believed to originate in Britain, were outlawed by Claudius, and in 61 they vainly defended their sacred groves from destruction by the Romans on the island of Mona (Anglesey).However, under Roman rule the Britons continued to worship native Celtic deities, such as Ancasta, but often conflated with their Roman equivalents, like Mars Rigonemetos at Nettleham.