Darius I's construction of Persepolis were carried out parallel to those of the Palace of Susa. According to Gene R. Garthwaite, the Susa Palace served as Darius' model for Persepolis. Darius I ordered the construction of the Apadana and the Council Hall ("Tripylon" or the "Triple Gate"), as well as the main imperial Treasury and its surroundings. These were completed during the reign of his son, Xerxes I. Further construction of the buildings on the terrace continued until the downfall of the Achaemenid Empire. According to the Encyclopædia Britannica, the Greek historian Ctesias mentioned that Darius I's grave was in a cliff face that could be reached with an apparatus of ropes.