The Romans had met the Druids before in conquered Western Europe. While the Romans were happy to make a peaceful settlement with most tribes/groups in England, they had no intention of doing the same with the Druids.
The Druids were priests. The Britons both respected and feared them. It was believed that a Druid could see into the future - they also acted as teachers and judges. They were considered to be very learned people. It could take up to twenty years of learning to become a Druid. However, we do not know a great deal about what they learned as Druids were not allowed to write any of their knowledge down.
In their own way, the Druids were very religious. It was this particular issue that angered the Romans as the Druids sacrificed people to their gods. Caesar, in particular, was horrified by the practice and his writings give us a good idea of what went on in Druid ceremonies -though from his perspective only. The Romans had once sacrificed people but they now saw it as a barbaric practice that they could not tolerate in one of their colonies. The Romans determined that they would stamp out the Druids.
However, they had to be careful. The Druids travelled freely throughout England as the Britons were too scared to stop them. Therefore, they were not simply in one place where the Romans could attack in force. In AD 54, the Emperor Claudius banned the Druids. In AD 60, the governor of England, Suetonius, decided that the only way to proceed was to attack the known heartland of the Druids - the island of Anglesey in the hope that if the centre of the Druids was destroyed, those Druids in outlying areas would die out.
Boats were built for the Roman foot soldiers while the Roman cavalry swam across with their horses. The Druids shouted abuse at the Romans and cursed them but they could not stop the Roman army from landing. Any ceremonial sites on Anglesey used by the Druids were also destroyed but many of them were in secret places and some survived.