Because the efficiency of the land transport system of this era was poor, the overwhelming majority of trade was local in scope. Economies based on autonomy and basic subsistence could not generate much trade. Cities were located to take advantage of the defensible or commercial advantage of a location. From the perspective of regional economic organization, the provision of cities in perishable agricultural commodities was limited to a radius of about 50 kilometers, at most. The size of cities also remained unchanged in time. Since people can walk about 5 km per hour and that they are not willing to spend more than one hour per day walking, the daily space of interaction would be constrained by a 2.5 km radius, or about 20 square kilometers. Thus, most rural areas centered around a village and cities rarely exceeded a 5 km diameter. The largest cities prior to the industrial revolution, such as Rome, Beijing, Constantinople, or Venice never surpassed an area of 20 square kilometers. Large cities above 100,000 were very rare and those who exceed such a population did so because they were at the nexus of maritime and land trade networks.
Prior to the industrial revolution, it was difficult to speak of an urban system, but rather of a set of relatively self-sufficient economic systems with very limited trade. The preponderance of city-states during this period can a priori be explained by transportation, in particular the difficulties of shipping goods (therefore to trade) from one place to another. Among the most notable exceptions to this were the Roman and Chinese empires, which committed extraordinary efforts at building transportation networks and consequently maintained control over an extensive territory for a long time period.
The Roman Empire grew around an intricate network of coastal shipping and roads. Its road network supported a set of large cities around the Mediterranean basin. It also traded with India and China.
The Chinese Empire established an important fluvial transport network with several artificial canals connected together to form the Grand Canal. Some parts of it are still being used today.