Following in the steps of Augustus, Caligula and Nero erected palaces on the Palatine. Caligula connected the hill with the Forum, and Nero opened up an entrance towards the Caelian. Vespasian built there the Flavian house which his son Domitian was to dedicate as the _Aedes Publica_, a gift to the people. Septimius Severus extended the Palatine towards the south by the construction of his Septizonium. Of the buildings of Tiberius, the columns of the temple of Ceres built into the church of S. Maria in Cosmedin remain to us; of those of Claudius, the beautiful ruined arches of his aqueduct. The Flavian emperors were great builders, and to this period belong the arch of Titus, built in A.D. 70 to commemorate the destruction of Jerusalem, a monument of Rome's best period, the ruined baths erected by this same emperor, and the great amphitheatre and ruins of the temple of Vespasian. Trajan's great buildings--his forum and triumphal arch, his basilica and library--are represented by a very small excavated portion of the basilica, and the column whose summit marks the height of the hill cut away by this emperor to make a roadway between the Quirinal and Capitol and thus relieve the congested traffic of the city. The only fragments left of the work of Hadrian are the ruins of a villa near Tivoli, the mausoleum and _Pons Aelius_, now the castle and bridge of S. Angelo; and behind the church of S. Francesca Romana in the Forum the ruins of the _Templum Urbis_, the temple of Venus and Rome, with its twin niches for the gods, one turned towards the convent the other looking outwards towards the Colosseum. The gilt bronze tiles from the roof of this temple were removed by Pope Honorius I. to deck the Christian _Templum Urbis_ S. Peter's. During the following 140 years there arose in Rome, amongst other monuments that have perished, the temple of Antoninus and Faustina built by Antoninus Pius in memory of his wife and now transformed into the church of S. Lorenzo in Miranda, the column of Marcus Aurelius, the triumphal arch of Septimius Severus dedicated to his sons Caracalla and Geta, the baths which bear this eldest son's name, although only begun by him and completed by Heliogabalus and Alexander Severus, the walls of Aurelian which still encompass the city and the _thermae_ of Diocletian. The latest of the imperial buildings were the temple built by Maxentius to his son Romulus, now the church of SS. Cosma and Damian in the Forum, and the baths, basilica, and triumphal arch of Constantine.