60, and of the two inner walls of the latter the first was 40 stades in length and the second 20.[500] The circuit of 22½ miles may, therefore, refer only to the whole extent of the citadel and city of Ecbatana, and it would then come near the statement of Diodorus, who puts the circuit of Ecbatana at 250 stades. The royal citadel of the Achæmenids at Persepolis was surrounded, according to Diodorus, with a triple wall; the first wall was 16, the second 32, the third 60 cubits in height. These walls were decorated with great expense, and adorned with towers. The citadel of Ecbatana is said to have had seven walls, and, however strange this may appear, religious no less than military motives might have led to that number. We know the importance ascribed in the Avesta to the number seven--the seven supreme deities, and the seven girdles of the earth. On the other hand, if we leave out of sight the hill on which the citadel was built, the elevated plateau of Ecbatana did not present the natural difficulties such as rivers and heights, which strengthened the fortification of the Assyrian metropolis and the walls of Babylon; hence it was more important to meet this deficiency by the number of towers and walls. The successive walls would have been of little service had not each inner one been higher than the outer, and they would have been the reverse of serviceable, had not the distance between them been so great that the arrows and missiles of the enemy, when they had gained the outer walls, failed to reach from it to the next, so that the assault of the next wall had to be begun afresh after the capture of the adjacent outer wall, without any cover. If we might assume that the words of Polybius: "The palaces, of which the extent reaches to about seven stades, lie below the citadel," are to be interpreted as meaning that these buildings lay under the protection of the walls of the citadel, _i.e._ behind them--and it is scarcely conceivable that the Achæmenids should have chosen an unprotected palace in Media for a constant residence, and deposited there large treasures, and the archives of the kingdom--the circuit of the buildings of the palace and that of the external wall of 60 stadia would give a space between the walls of each of about 2000 feet; a distance not much in excess of that absolutely required. Hence the circuit of 60 stadia from the outer wall does not appear too large.