When Edward I took Caernarfon, seat of the Princes of Gwynedd, in 1278, he conquered the last remaining part of the Roman Empire. Why was it that this part of the Roman Empire survived for 800 years before succumbing to the medieval kingdoms that succeeded Rome? In answering this question, this book offers a new and innovative perspective on Wales, the South West and the Welsh Marches at a time when they were united as Britannia Prima, one of the four late Roman provinces of Britain. Created at the end of the third century, the province endured for 200 years, offering a successful resistance to the incoming Anglo-Saxon invaders throughout the fifth century. This book, the first ever to examine one of the provinces of late Roman Britain, provides a critical analysis of the abundant archaeological information now available. In doing so it paints a picture of a wealthy and flourishing Roman society in the fourth century, able to achieve a measure of economic self-sufficiency through its wealth of natural resources, a society that maintained its Roman, and urban, character throughout the fifth century. Eventually Britannia Prima fragmented, overwhelmed by the internal and external pressures that it faced, but its enduring legacy is the distinct nature, culture and identity of the Welsh and Cornish kingdoms that succeeded it.