The City Walls are the defensive walls of the ancient city. The First Circle was erected between the third and second centuries BC, in opus quadratum and was interested in the top of the Poggio Hill, assuming an elliptical shape. In these were opened four Gates, corresponding to the two main roads of the Roman city and the east-west Gates. The Second Circle was built still only on the Poggio between the twelfth and thirteenth centuries, to encompass the villages that sprang up immediately outside the ancient doors of the Roman era. The Third Circle was built by Duke Federico da Montefeltro III in the second half of the fifteenth century, to adapt the defensive City Walls to the urban changes of the period: the construction of the Ducal Palace and the Cathedral. The Fourth Circle, current Wall of the old town, is almost completely preserved and restored. It was built in the first half of the sixteenth century by the Duke Francesco Maria I della Rovere, to adapt the old City Walls to the nascent artillery, with an embankment and protruding heart-shaped bastions, including the Belisario Bastion, to the southeast, restored to host part of the buildings of the old Barriera Margherita.