The historic center of Ferentino is enclosed within Cyclopean walls, on account of the size of the boulders, that date back to the sixth century BC. In medieval times the walls were reinforced with towers made of irregular stones. The building circuit winds around for 2.4 Km with 12 gates and its origin is attributed to the Pelasgians, the ancient local population. Built with local limestone, rough and difficult to chisel, it was known to the ancients by the name of silex. Surprisingly, the underground route to the Grotta Para is completely architraved (roofed) with large Cyclopean stones.