At the beginning of the article, we explore the “harbour-city” connection of the Ionian and Carian coastal urbanisation at its highest point in Antiquity. The suitable conditions of the environment caused a settling next to the harbour. We therefore examine the socio-cultural structure by the elements of “harbour + trade + religion + city” which resulted in this urbanisation. In relation with these remarks, we examine the interlacing of sea trade with land trade, the migration and colonisation of this particular period. Further on, we examine the “harbour-city” juxtaposition which resulted in the grid plan of Hippodamos in Miletos. We also explore the topography; the influences of the Eastern world; the necessity of reconstructing the city; the new ideas which were contributed by the “nature philosophers” and the political conception of equality. The conclusion part consists of the appraisal of the accumulated facts. By focusing on the example of Miletos, we investigated the architectural structure and formation of the Ionian and Carian harbour cities. We emphasize that, in addition to the advantages of a geographical location, political and religious beliefs also have a strong influence on the establishing of a city.