The ancient walls surround the historic city almost without interruption for nine kilometres. This makes them some of the oldest and most impressive defensive systems of the Middle Ages. The builders made use of all the techniques of defence available in the 1400 and 1500s, with city gates, bastions, large towers, gun slots and embrasures. Built in 1612 from a design by Giovan Battista Aleotti, the Porta Paola stands at southern entrance to the walls, when Ferrara was devolved to the Papal States. The two bastions to the west were commissioned by Pope Paul V.
