The castle of Civitella San Paolo is one of the best examples of medieval military architecture and is perfectly preserved. The ‘keep’ dates from the tenth or eleventh century and had a lookout on the river traffic functions of the Tiber valley. In the fourteenth century it was expanded to a large four-sided fortress surrounded by wide moat and drawbridge, by the monks of St. Paul. In the fifteenth century the walls were strengthened to defend against the power of firearms, and a pentagonal bulwark was added facing the current Piazza San Giacomo. The castle formed a single system with the walls, towers, turrets and ramparts that surrounded the town. Finally, in the second half of the fifteenth century, the residential palace was built that includes the church of Santa Maria, which had open windows instead of the original narrow openings. The Monastery of St. Paul was under the protection of the Papal States so the battlements of the castle are Guelph, different from the dovetail design of the Ghibellines. Access from the town was possible through Porta Capena. The residential palace is now the town hall.