The earliest of the great Southern palaces of East Baghdad, where, during the last four centuries of the Abbasid dynasty, the Caliphs held their court, had originally been a pleasure house, built by Ja'far the Barmecide, brother-in-law and boon companion of Harun-ar-Rashid. Mamun, the heir-apparent, from the time of his birth had been put under the nominal guardian-ship of Ja'fari, and the Caliph graciously accepting the gift for his son, the new palace, at first called the Ja'fari, came afterwards to be known as the Mamuni, though it remained exclusively in the occupation of Ja'far until the fall of the Barmecides. With the accession of Mutadid and the permanent establishment of the Caliphate in East Baghdad, a new era of palace-building is inaugurated, for this Caliph not only enlarged the Hasani and laid the foundations of the Taj, but built for himself two other palaces, namely the Firdus and the Thurayya.