The Tower of Nicholas II was built with a bull issued by Pope John XXIII in November 970 and it represents the nucleus of a Castrum. The tower is all that remains of the triple ring of walls that surrounded the town. The name comes from the pope, who stayed in the castle that was defended by the inhabitants of Roccantica. The tower has a square section and is inserted in the second set of walls that had three doors: the Door (or Porta Rieti), Porta Nuova and Porta dell'Arco. Of the first circle only the buttresses survive, while the third circle was completely incorporated into the houses of the old town.