Historians divide the past into periods on the basis of economic and social factors to characterise the major elements of different moments of the past. Yes, the historians face a lot of problems because economic and social factors do not exist in a stasis, they have a tendency to change constantly.
Roman cities were made mostly out of stones and, they had long and wide paved streets, they often had a sewage system. This wasn't the case (any of the three differences) for cities during the European dark ages.
