The 80 books of Roman History span from the myth of the city's founding by Aeneas to the year 229 AD. As a high-ranking state official, Cassius Dio was able to utilize not only the 28 public libraries of Rome but also the archives of the Senate and the imperial household when composing his work. In the Senate, he witnessed the political and historical events of the decades from Emperor Commodus (180 to 192) to Alexander Severus (222 to 235) up close. Particularly, the accounts of the imperial period in Cassius Dio's reports are of invaluable worth, as he narrates as a contemporary witness.