Near the Porta Pia, to the north-east of the church of S. Maria degli Angeli, the square of the Castra Prætoria projects from the Avails. This permanent camp was established under Tiberius by Sejanus; subsequently Aurelian made use of the four outer walls of this camp as a part of his fortification, and therefore Constantine, when he abolished the Prætorian Guard, pulled down the side towards the city only. The porta decumana of the camp is still to be seen though it is now walled up, and also the porta principalis dextra, but the porta principalis sinistra has disappeared or perhaps never existed. The camp was enclosed by a wall at least as early as the time of Pertinax and Julian, for here occurred that memorable and most melancholy scene in Roman history, when the Prætorian guards shut themselves within their camp after the murder of the reforming Emperor Pertinax and put up the throne to auction. Julian and Sulpicianus were the bidders. The soldiers let down a ladder and allowed Julian to get up upon the wall, says Herodian, for they would not open the gates before they heard how much would be offered. Sulpicianus was not allowed to mount the wall. They then bid one against the other, and at last they ran up the price little by little to five thousand drachmas to each soldier. Julian then impatiently outbid his rival by offering at once six thousand two hundred and fifty, and the Empire was knocked down to him. This was not by any means the first or only time that the fate of the Empire had been decided here.[111] The chief power in the Roman state had lain within these walls of the Prætorian camp since the time when Tiberius consented to allow their designing colonel, Sejanus, to establish the Prætorian guards in permanent quarters, and the readers of the historians of the Empire will recall the many vivid pictures of their rapacity and violence. To go to the Prætorian camp and promise a largess to the guards was the first duty of a Roman emperor. The eastern side of the camp, which is probably the only one now retaining its original form, measures 500 yards, and the southern 400 yards. The latter seems to have been partly pulled down, and the northern side has also been altered. Aurelian’s Wall did not exactly meet the two angles of the camp towards the city, but its course was here determined by the houses and buildings in the vicinity which it was desirable to protect. The walls of the camp were, according to Bunsen, at first only fourteen feet high, but were raised by Aurelian and fortified with towers. Some parts of the walls doubtless consist of the original brickwork of Aurelian’s time, as the masonry bears the marks of great age, and is of a most regular and solid style. A few of the soldiers’ quarters are still left, consisting of rows of small low arched rooms similar to those on the Palatine and at Hadrian’s Villa near Tivoli.[112] [Sidenote: Porta Chiusa.]