The land became overran by barbarian tribes. The old Roman cities, roads, aqueducts, fell into disrepair. Some barbarian groups didn't stop their ransacking there they continued on, the Vandals went as far as Northern Africa, and the Huns merely continued to plunder the Eastern Roman Empire. Some barbarian groups however settled. Like the Franks, Goths, Angle-Saxons, who merely moved onto empty lands built their settlements.
