The holes which are now so conspicuous in the travertine blocks of the exterior wall of the Coliseum were probably made in the middle ages to extract the iron clamps by which the stones were fastened together. Some antiquarians have however held that they are the holes in which the beams of the buildings which clustered round the Coliseum in mediæval times were fixed. At the end of the fifteenth and at the beginning of the sixteenth century, the travertine blocks of the amphitheatre were used as a quarry from which to build palaces, and it is said that the Palazzo di Venezia, the Palazzo Farnese, and the Palazzo della Cancelleria were constructed of the stone robbed from hence. During part of the eleventh and twelfth centuries a castle of the powerful family of the Frangipani, which afterwards belonged to the Annibaldi stood in the walls of the Coliseum. Later generations of nobles and popes since the beginning of the nineteenth century have propped the building by buttresses of brickwork, and have endeavoured to postpone the date foretold by two Anglo-Saxon pilgrims as that of the fall of Rome. “When the Coliseum falls, Rome will fall.”[73] [Sidenote: Aurea Domus of Nero. Baths of Titus.] The Coliseum was built by Vespasian in a depression between the Cælian and Esquiline Hills, which had been occupied by a large lake of ornamental water, called the Stagnum Neronis, used by Nero for aquarian entertainments and exhibitions. The vast palace known as the Domus Aurea Neronis extended along the side of the Esquiline on the north of the Coliseum. The Flavian emperors destroyed this palace, and Titus built a new group of courts and chambers over the ruins. The relics of these buildings of Titus are now remaining mingled with the substructures and lower parts of the Domus Aurea which they superseded. They are entered by a gateway on the road leading from the Coliseum to S. Pietro in Vincoli.