The cohors praetoria existed by the 2nd century BC, acting as bodyguards for Roman generals. They were formally organized in the time of Augustus, he created a permanent corps of nine cohorts (from 500 to 1,000 men each). The number of cohorts forming the guard varied over time. The Praetorians, led by a prefect, attended the emperor wherever he went. They had special privileges, where paid much more then ordinary troops and after their services they got a severance pay. In 23 AD Tiberius' powerful prefect Sejanus became their sole commander, he concentrated them in fortified barracks outside the walls of Rome, gaining significant political influence for them. In the period when the empire declined, the praetorian guard held almost unchallenged authority. Constantine I disbanded them in 312 AD.