The first core part of the castle was built in the early Middle Ages on a rocky outcrop where the population had taken refuge. The castle then had a transformation in the firth century and again in six hundreds to make it into a palace. Today it is well preserved and you can see the main features of the castle: the city walls, 'last stand' tower, and palace. The walls are protected by five rectangular towers. At the north it borders the deconsecrated St. Mary's Abbey 'trans pontem' (across the Tiber bridge). It is accessed via Porta Capena, a typical Renaissance round arch worked in earthenware ashlar, in which we recognize the traces of a drawbridge. In the first renovation of 1493, Nicholas III Orsini had the castle enlarged by adding a new wing with nine rooms. In one of these, the Hall of the Guard, Pope Alexander VI Borgia celebrated December 19, 1493 the ceremony of the 'concession of indulgences' as recorded on a large plaque. Today this room is used by the city for meetings and conferences and to celebrate weddings. Finally, in the eighteenth century Marco Ottoboni added a new wing. From the large terrace, surrounded by Guelph battlements, you can enjoy a spectacular view across the valley.