The aim of this paper is to dig deeper in order to gain a better understanding of the territorial logic of Roman roads, following some recent approaches based on the use of digital modelling tools. Taking the case of the so-called via XVII (a ca. 330 km itinerary which joined Bracara Augusta and Asturica Augusta, NW Iberian Peninsula), the paper explores various factors, both natural and cultural, which may have been determinant in the layout of main roads in the Roman period. This study has followed a non-reconstructive methodology based on the theoretical idea of "least-cost paths" implemented by way of GIS tools. The analysis combines different variables which have an impact on human mobility (such as terrain slope and altitude, and the a priori existence of some primary nodes) and different spatial scales of analysis. As an outcome, we have achieved a detailed understanding of the factors behind the layout of this road and, in doing so, we have also drawn some conclusions regarding the historical context of its origin and development. The methodology and some of the results may be pertinent for the analysis of Roman roads elsewhere and, to some extent, for the wider analysis of ancient roads.