In 23, Sejanus convinced Tiberius to have the Castra Praetoria (the fort of the Praetorians) built just outside of Rome. Through the machinations of their ambitious prefect, Lucius Aelius Sejanus, the Guard was brought from the Italian barracks into Rome itself. This system was not radically changed with the appointment by Augustus in 2 BC of two Praetorian prefects, Quintus Ostorius Scapula and Publius Salvius Aper, although organization and command were enhanced. While they patrolled inconspicuously in the palace and major buildings, the others were stationed in the towns surrounding Rome no threats were possible from these individual cohorts. turma) of 30 men each were also organized. A small number of detached cavalry units ( turmae, sing. Thus he allowed only nine cohorts to be formed, originally of 500, then increased to 1,000 men each, and only three were kept on duty at any given time in the capital. While Augustus understood the need to have a protector in the maelstrom of Rome, he was careful to uphold the Republican veneer of his regime. The group that was formed initially differed greatly from the later Guard, which came to be a vital force in the power politics of Rome.
Thus, from the ranks of the legions throughout the provinces, Augustus recruited the Praetorian Guard. When Augustus became the first ruler of the Roman Empire in 27 BC, he decided such a formation was useful not only on the battlefield but in politics also. As Caesar discovered with the Legio X Equestris, a powerful unit more dangerous than its fellow legions was desirable in the field. In time, this cohort came to be known as the cohors praetoria, and various notable figures possessed one, including Julius Caesar, Mark Antony and Augustus (Octavian). They consisted of both infantry and cavalry. It was a habit of many Roman generals to choose from the ranks a private force of soldiers to act as guards of the tent or the person. They were an elite recruitment of Roman citizens and Latins. The term Praetorian derived from the hut of the commanding general or praetor of a Roman army in the field-the praetorium.