The landscape of Lazio’s Tyrrhenian coasts is strongly characterized by the presence of fortifications. Parallel to them, in the interland, the baronial expansion, between the tenth and eleventh centuries, paved the way to a large-scale fortification of the Roman countryside. Along the main routes were built lookout towers, farmhouses were consolidated with defense mechanism and the first castles were constructed. The research focuses on the study of Castellaccio di Monteroni in Ladispoli, built in the fifteenth century on Roman structures and on previous constructions dating back to the period of the Baronial expansion. The Castellaccio di Monteroni is one of the few remaining examples of Casale Fortificato (Fortified Manor). It is placed on the 35th kilometer of the Via Aurelia and takes its name Castellaccio from the abandonment over the centuries and Monteroni due to the heaps of the nearby Etruscan necropolis. The main function carried out over the centuries was a resting place, a sure point of reference for couriers, travelers and pilgrims traveling along the Via Aurelia. When in the nineteenth the route of the Via Aurelia was moved to the present one the fortification fell into abandonment. The analysis of this architecture is of great interest, not only for the knowledge of the geometry of the fortifications of the Roman countryside, but also because it is one of the few remaining examples of Casale Fortificato, representing a rare medieval architectural heritage.