Forte Sangallo, Renaissance military fort in Nettuno, Italy.

Forte Sangallo is a quadrangular Renaissance fortress with very thick, uncrenellated walls and a high scarp, built from 1501 for Pope Alexander VI and Cesare Borgia. At its corners are bastions with chamfered angles called orecchioni, and two tiers of covered artillery positions (troniere) are hidden in the gorge between curtains and bastions. A central keep was enlarged later.

Pope Alexander VI and his son Cesare Borgia had Forte Sangallo built from 1501 to defend Nettuno, then called the "breadbasket of Lazio", from Barbary corsair raids by sea. The design is attributed to Antonio da Sangallo the Elder, though Giuliano da Sangallo and Baccio Pontelli have also been proposed. After the Borgias it passed to the Colonna in 1503, the Apostolic Camera in 1594 and the Borghese in 1831; the 1925 Convention of Nettuno between Italy and Yugoslavia was signed here.

Since 1997 the fort has belonged to the municipality of Nettuno, which uses it for conferences and contemporary art exhibitions. It also houses the Museo dello Sbarco Alleato, devoted to the Allied landing at Anzio, and an Antiquarium displaying archaeological and artistic finds from the Nettuno area. The Guerre & Pace FilmFest is held here.

Access is usually by guided tour, which explains the defensive systems and how the fort works. Wear sturdy shoes since you will climb stairs and walk on uneven stone surfaces.

The fort's two tiers of covered gun positions, called troniere, sit hidden in the throat between the curtain walls and the corner bastions. The Da Sangallo model may have been used here for the first time, giving complete grazing fire along the walls. The corner bastions have chamfered angles known as orecchioni.