Forte Sangallo is a well-preserved Renaissance fortification in Nettuno, the twin town just east of Anzio. Built in the late 15th and early 16th centuries under Pope Alexander VI, it was designed to defend the coast from Ottoman naval raids — a real concern at the time, given the range of Barbary corsairs in the Tyrrhenian Sea. The fortress is named after Giuliano da Sangallo, the architect who worked on it, though his involvement is debated.
The fort is compact and impressive, with circular towers at the corners and a deep moat. It functions today as a museum and cultural space and can be visited. The historic centre of Nettuno beside it, with its medieval lanes and fish restaurants, makes this a worthwhile half-day addition to an Anzio visit.