Perhaps the most intriguing facet of Gallic religion is the practice of the Druids. The druids presided over human or animal sacrifices that were made in wooded groves or crude temples. They also appear to have held the responsibility for preserving the annual agricultural calendar and instigating seasonal festivals which corresponded to key points of the lunar-solar calendar. The religious practices of druids were syncretic and borrowed from earlier pagan traditions, with probably indo-European roots. Julius Caesar mentions in his Gallic Wars that those Celts who wanted to make a close study of druidism went to Britain to do so. In a little over a century later, Gnaeus Julius Agricola mentions Roman armies attacking a large druid sanctuary in Anglesey in Wales. There is no certainty concerning the origin of the druids, but it is clear that they vehemently guarded the secrets of their order and held sway over the people of Gaul. Indeed, they claimed the right to determine questions of war and peace, and thereby held an "international" status. In addition, the Druids monitored the religion of ordinary Gauls and were in charge of educating the aristocracy. They also practiced a form of excommunication from the assembly of worshippers, which in ancient Gaul meant a separation from secular society as well. Thus the Druids were an important part of Gallic society. The nearly complete and mysterious disappearance of the Celtic language from most of the territorial lands of ancient Gaul, with the exception of Brittany, can be attributed to the fact that Celtic druids refused to allow the Celtic oral literature or traditional wisdom to be committed to the written letter.