The Lateran Palace formally the Apostolic Palace of the Lateran is an ancient palace of the Roman Empire and later the main papal residence in southeast Rome. Located on St. John's square in Lateran on the Caelian Hill, it is adjacent to the Archbasilica of St. John Lateran, the cathedral church of Rome. From the 4th century, the palace was the principal residence of the popes, and continued so for about a 1000 years until the seat ultimately moved to the Vatican. The palace is now used by the Vatican Historical Museum, which illustrates the history of the Papal States. The palace also houses the offices of the Vicariate of Rome, as well as the residential apartments of the Cardinal vicar.