Built by the Romans in the 3C, these walls were modified in the Middle Ages. Made from schist slabs, and rising to a height of 11m, they encircle the town for a distance of over 2km, punctuated by ten gateways providing access to the old quarter. Magnificently preserved, they were designated a UNESCO World Heritage Site in 2000. Make sure you climb up onto the walls, from where there are fine views of the town and the surrounding countryside.