Complex sentence: The Britons (sometimes Brythons or British) were a Celtic people who lived in Great Britain and spoke the Insular Celtic language known as British from the British Iron Age until the Early Middle Ages.
Split into simpler sentences: The Britons (sometimes Brythons or British) were a Celtic people who lived in Great Britain and spoke the Insular Celtic language known as British. Their culture held sway in Britain from the British Iron Age until the Early Middle Ages.