The aim of the work is to shed light on the problem of the location of the ancient Castellum Firmanorum, the actual naval port of Firmum Picenum. Although there are references to it in various ancient historical sources, both literary and epigraphic, its location still remains largely unresolved. Starting from the “status quaestionis”, the work gathers together all the historical and archaeological data which are currently available, divides them into homogenous categories, and assesses them critically, supplementing them with accurate surface and aerial topographic observations. The analysis of the data collected in this way leads the author to place the river port of Firmum Picenum at the mouth of the present day River Ete, in the district of Salvano di Fermo, where there are significant archaeological remains. The historical analysis of this proposed location of the naval castle is further supplemented by an extensive “documentary appendix” of complete, previously unpublished, archive material. These include an interesting manuscript by Filippo Eugenio Mecchi, an incisive academic from Fermo, and information about pre-nineteenth century archaeological finds. The work ends with a practical index of names and important items cited in the text.