When maritime transport and communication depended on muscle and wind-power, the Mediterranean Sea functioned as a symbiotic force between the civilisations which surrounded it, at once the major dividing barrier and the major connecting element. In this study, the technological limitations of maritime traffic are considered in conjunction with the peculiar geographical conditions within which it operated, and which led to the establishment of major sea lanes on trunk routes along which traffic could move safely, efficiently, and economically. These trunk routes remained virtually unchanged from antiquity to the sixteenth century, and eventually constituted economic and strategic maritime frontiers between civilisations. At the same time, the technological limitations of the oared galley meant that coasts and islands along the trunk routes had also to be held, a necessity which favoured geographically the Christian West over the world of Byzantium and Islam.