The Castra were erected just outside Rome by order of the Praetorian Prefect Sejanus, between 20 and 23 AD, during Emperor Tiberius' reign, to house the nine cohorts of the Praetorian Guard, the influential Imperial guard. When Emperor Aurelian encircled the city with walls, he had the barracks incorporated in the fortification. After Constantine's victory over Maxentius in 312, he disbanded the Guard and destroyed the barracks, as Praetorians had been fighting on Maxentius' side.