Xerxes, dissatisfied with the size of the old summer palace, built a new one at its side of considerably larger dimensions, using perhaps some of the materials of the old palace in his new building.

Finally, Artaxerxes Ochus made certain additions to the palace of Xerxes on its western side, and at the same time added a staircase and a doorway to the winter residence of Darius.

Thus the Persepolitan palace, using the word in its proper sense of royal residence, attained its full dimensions, occupying the southern half of the great central platform, and covering with its various courts and buildings a space 500 feet long by 375 feet wide, or nearly the space covered by the less ambitious of the palaces of Assyria.
Besides edifices adapted for habitation, the Persepolitan platform sustained two other classes of buildings.

These were propylaea, or gateways--places commanding the approach to great buildings, where a guard might be stationed to stop and examine all comers--and halls of a vast size, which were probably throne-rooms, where the monarch held his court on grand occasions, to exhibit himself in full state to his subjects.

The propylaea upon the platform appear to have been four in number.