It is important not to lose sight of the fact that the palaces, the elements of which we have just described, constitute an official kind of architecture implanted in Persia by the “kings of kings” who were pleased with the monuments that they had observed in Egypt, Assyria, and Asia Minor. Springing from the caprice of sovereigns, this foreign architecture never took root in the country, and was not required by the nature of the ground and the necessities of existence on the mountainous table-land of Persia; it disappeared with the Achæmenid dynasty. But by the side of this conventional architecture there was that created by the natives of the country, because it had been imposed upon them as a condition of life. As well as the people of Chaldæa and Assyria, the Persians must have known how to build vaulted houses, alone capable of protecting them from the rays of a too ardent sun; they also built, at least in the cantons of Susiana, houses with terraces, supported by palm beams and trellis-work arranged over the rooms which were narrow like passages. Strabo tells us this while speaking of Susiana: “To protect the rooms from the excessive heat, the roofs are covered with two cubits’ depth of earth; the weight of this earth obliges the people to build all the houses long and narrow, because, although the beams must not be very long, nevertheless the rooms must be spacious; otherwise the people would be stifled.” Even at the present day, since the climatic conditions of the country have not changed, the method of building houses is the same as that practised by the ancient inhabitants of Iran. Travellers find houses, according to the wealth of the owner, surmounted by vaults, domes, and terraces, wonderfully suited to local requirements. It is, then, quite certain that the Iranians in the time of the Achæmenids knew the vault and the cupola as well as their neighbours on the banks of the Tigris.