The following is a discussion on the structure and phases of the Castra Praetoria, the barracks of the Praetorian Guard, built in AD 21 to 23 under the auspices of Tiberius Caesar’s praetorian prefect, Lucius Aelius Sejanus, to concentrate the nine Praetorians cohorts together with three Urban Cohorts at one location within close proximity to Rome’s centre, as related by Cassius Dio.