Port cities are places sui generis. They are starting points for opening up the world both militarily and economically, sites of intersection between land and sea trade, for the local, regional and trans-regional exchange of goods, and are thus places characterized by economic activities, migration and cosmopolitanism. In short, they are ‘urban agglomerations of human mobility’. Roman port cities were provided not only with the usual elements of infrastructure and architecture, designed to impress with their monumentally staged regularity, but also with extravagantly planned ‘waterfronts’ (see Figure 12 in Chapter 2): the orthogonal network of streets; aqueducts, fountain buildings and thermal baths; the centrally located sanctuaries dominating squares, columned streets leading towards them; the towering theatre buildings which, tall and massive, caught the eye from the outside across the houses of the cities; the solid fortifications, completed by harbour basins, breakwaters and gigantic piers; a lighthouse, sometimes made of white marble; and broad coastal roads with inlets to house the crews of incoming ships. Greater than life-sized statues flanked the entrances to the harbours, providing safety from the incalculable sea but also from piracy. At the same time, however, a port city had the function of a gate, of both a ‘sally port’ and a ‘gateway’, predominantly the latter – for merchants, members of the armed forces and the Imperial administration, magistrates and subaltern public officials; for travellers, visitors, pilgrims, artists and scholars; for migrants who were not there for short stays but intended to stay longer, perhaps for good. They completed the society of a municipium or a provincial capital, of which we commonly know only of the élite – that is to say, those who dominated the political discourse and also, due to the donations and honours they received, the public spaces, as they occupied the prestigious offices or key positions of the socio-economic network of relationships, to form ‘sub-élites’, such as those of the members of cultic and professional associations: ingenui, peregrini and liberti. Thus, port cities had social structures that were both much more differentiated and more ethnically mixed than those of other cities.