Yet the Romans still constructed a fort to protect the city, called Castrum Rauracense, around 300 A.D. Though Roman emperors used Castrum Rauracense as a military base to beat back invaders ...

which comes from the Latin word castrum, meaning 'fortress'. And in Wales, many place names that start with ‘Caer' or 'Car’ - meaning ‘fort’ - refer to the Roman forts that were built there.