The Forte Sangallo is the pentagonal Renaissance fortress that dominates the northern edge of Civita Castellana's old town. Pope Alexander VI commissioned it in the 1490s as a combination garrison and papal refuge, and Antonio da Sangallo the Elder completed the main structure around 1500. The design is a textbook of late-15th-century military thinking: five angular bastions project outward to eliminate blind spots, a dry moat rings the outer walls, and the whole thing sits on the edge of the tufa plateau where the ravine provides a natural additional defence. It is one of the most complete and best-preserved examples of Renaissance military architecture in Lazio.
The fortress passed through the hands of various papal governors and commanders over the centuries and eventually became a barracks. Today it houses the Museo Archeologico dell'Agro Falisco, which alone is reason enough to visit. The building itself rewards a careful look: the stonework is unusually refined for a fortification, the proportions are elegant, and the pentagonal plan — unusual in Italy at the time — gives it a distinctive silhouette from every angle.