Praetorian Guard was established under the Roman Republic to guard the commander (praetor). Augustus raised the strength of the cohors praetoria to three cohorts, and later to nine with a body of horse. Tiberius brought the whole force to Rome, and established them in the castra praetoria. Vitellius formed sixteen cohorts of 1,000 each, and the praetorians received double pay, many privileges of extra rank, and the like, and a gratuity of £150 upon retiring after 12 to 16 years' service. The Praetorian Prefect was a man of much importance, and the whole body became a formidable political power, making and unmaking emperors at will. They were abolished by Constantine the Great in 312 A.D.