Complex sentence: They also left many artifacts, which have been discovered in increasing numbers by archaeologists in recent times.They were the only peoples who founded new cities in western Europe after the fall of the Roman Empire but before the rise of the Carolingians.
Split into simpler sentences: They also left many artifacts, which have been discovered in increasing numbers by archaeologists in recent times. They founded the only new cities in western Europe from the fall of the Roman Empire until the rise of the Carolingians.