Sources: Cities: Hanson, J. W. (2016). Cities Database (OXREP databases). Version 1.0. Ports: de Graauw, A. et al. 2014. “Geodatabase of Ancient Ports and Harbors,” DARMC Scholarly Data Series, Data Contribution Series # 2013-2 (version 1.1). DARMC, Center for Geographic Analysis, Harvard University, Cambridge MA 02138. Roads: McCormick, M. et al. 2013 – Roman Road Network (version 2008).
About 3,000 ports were identified throughout the Roman Empire, mostly around the Mediterranean (Mare Internum). However, many ports were only rudimentary harbors where ships could anchor. The above map depicts ports that had a breakwater or a lighthouse, which represent the first comprehensive port infrastructures where the natural advantages of a site are improved. Due to the high costs of such improvements, the amount of traffic must be sufficient enough to justify their construction, repair, and maintenance.
The density of the ports is a function of the economic density of the Roman Empire with clusters around the Aegean Sea (with the major city of Byzantium), the Roman coast (Western Italy, such as Ostia and Portus), Southern Gaul, Northern Africa (around Carthago and Alexandria), and the Levant. This represented one of the first large scale integrated maritime shipping networks. However, limited information is available about the cargo handled by the ports of that era, but active trades in grain, wine, olive oil, and minerals were reported in historical documents. The security of these trade routes also required a permanent navy. It is not surprising that the shape of the Roman Empire is influenced by the shape of the Mediterranean (Mare Internum) since maritime shipping was the only mode of that era that allowed the long-distance trade of bulky and lower value commodities.