Fort Sangallo was built starting from 1501 and quickly erected by pope Alexander VI and his son Cesare Borgia, inside a vast program for strengthening of the fortifications of the places that it was intended to ensure the signoria hereditary family and, in particular, of the duchy of Sermoneta. The strong defended Neptune, at the time considered "granary of Lazio", from attacks by sea. After the Borgia, the fortification was passed to the column and then to the Apostolic Chamber and, in the nineteenth century, to the Borghese. In 1925 came here concluded the Convention of Neptune, between Italy and Yugoslavia. In 1931 the strong belonged to baron Fassini, that you hosted the Queen Mary of Romania and the princess Ileana. Today the building, owned by the town hosts conferences and exhibitions of art, the "Museum of the allied landing" and the Antiquarium, which exposes the material archaeological, historical and artistic value of the territory of Neptune.
The strong, located in the vicinity of the sea and to the west of the medieval village of Neptune, has a quadrangular shape: at the vertices, bastions with bevelled corners said "mumps"; walls very thick without battlements and a high shoe; two superimposed rows of seats of artillery blankets (troniere), hidden in the intersection (groove) between curtain walls and bastions, according to an innovative model due precisely to the Sangallo, (and maybe adopted here for the first time in order to ensure complete coverage of the shot grazing to protect the walls). At the center, an imposing donjon was expanded in successive times.