Following the map of ancient Rome under the empire we may note the more important monuments which still exist in something like their original condition. Across the Tiber and beyond the Campus Martius stands the mausoleum of Hadrian. [56] The most notable structure in the Campus Martius is the Pantheon. [57] It is the one ancient building in the entire Roman world which still survives, inside and out, in a fair state of preservation. The depression between the Caelian and Esquiline hills contains the Flavian Amphitheater, better known as the Colosseum. [58] It was begun by Vespasian and probably completed by Titus. No less than eighty entrances admitted the forty-five thousand spectators who could be accommodated in this huge structure. Despite the enormous mass of the present ruins probably two-thirds of the original materials have been carried away to be used in other buildings. Close to the Colosseum stands the arch [59] erected by the Senate in honor of the victory of Constantine over his rival Maxentius. From this event is dated the triumph of Christianity in the Roman state. The ruins of the huge baths of Caracalla lie about half a mile from the Colosseum. Near the center of the city are the remains of the Forum added by Trajan to the accommodations of the original Forum. It contains the column of Trajan [60] under which that emperor was buried. THE FORUM The Forum lies in the valley north of the Palatine Hill. It was the business and social center of the Roman city. During the Middle Ages the site was buried in ruins and rubbish, in some places to a depth of forty feet or more. Recent excavations have restored the ancient level and uncovered the remains of the ancient structures. [Illustration: THE ROMAN FORUM AND THE SURROUNDING BUILDINGS (RESTORED)] [Illustration: THE ROMAN FORUM AT THE PRESENT TIME] APPROACH TO THE FORUM The Forum could be approached from the east by one of the most famous streets in the world, the Roman Sacred Way. The illustration of the Forum at the present time gives a view, looking eastward from the Capitoline Mount, and shows several of the buildings on or near the Sacred Way. At the left are seen the ruins of the basilica of Constantine. Farther in the distance the Colosseum looms up. Directly ahead is the arch of Titus, which commemorates the capture of Jerusalem in 70 A.D. [61] The ruins of the palaces of the Caesars occupy the slopes of the Palatine. THE FORUM TO-DAY The only well-preserved monument in the Forum is the beautiful arch erected by the emperor Septimius Severus. Beyond it are three columns which once formed part of the temple of Castor. They date from the time of Tiberius. In front are the foundations of the Basilica Julia, built by Augustus. Next come eight Ionic columns, all that remain of the temple of Saturn. Near it and in the foreground are several columns in the Corinthian style, belonging to a temple built by Vespasian. THE FORUM IN ANTIQUITY