This paper investigates the potential of frontiers to shape the economic geography of countries. I focus on the case of Spain to explore how historical frontier warfare affects the colonization of the territory and the distribution of the population across the space. Exploiting a spatial discontinuity in military insecurity during the Christian colonization of central Spain in the Middle Ages, my findings suggest that medieval frontier warfare heavily conditioned the settlement of the territory, resulting in a sparse occupation of the space, low settlement density and high population concentration. These initial features of the colonization process were already visible in the early 16th century and have persisted to this day, with potential negative consequences for economic development.