Groups of Belgae and Celts, known as Gauls, invaded Normandy in the 4th and 3rd centuries BC. Much of our knowledge about this group comes from Julius Caesar’s book "de Bello Gallico". Caesar wrote about many groups among the Belgae who lived different areas in towns surrounded by walls. In 57 BC the Gauls worked together and tried to fight Caesar’s army. They were led by Vercingetorix. The Normans lost an important fight at Alesia. They did not stop fighting until 51 BC, when Caesar finished his conquest of Gaul. Normandy was part of Armorica when it was ruled by Romans.