Being at length convinced that Alexander would not be satisfied with any prize short of the entire Persian empire, Darius summoned all his forces to defend what he still retained. He brought together a host said to be superior in number to that which had been defeated at Issus.[368] Contingents arrived from the farthest extremities of the vast Persian territory—from the Caspian sea, the rivers Oxus and Indus, the Persian Gulf, and the Red Sea. The plains eastward of the Tigris, about the latitude of the modern town of Mosul, between that river and the Gordyene mountains (Zagros), were fixed upon for the muster of this prodigious multitude; partly conducted by Darius himself from Babylon, partly arriving there by different routes from the north, east, and south. Arbêla—a considerable town about twenty miles east of the Great Zab river, still known under the name of Erbil, as a caravan station on the ordinary road between Erzeroum and Bagdad—was fixed on as the muster-place or head-quarters, where the chief magazines were collected and the heavy baggage lodged, and near which the troops were first assembled and exercised.[369] [368] Arrian, iii. 7, 7. [369] Diodorus, xvii. 53; Curtius, iv. 9, 9. But the spot predetermined for a pitched battle was, the neighborhood of Gaugamela near the river Bumôdus, about thirty miles west of Arbêla, towards the Tigris, and about as much south-east of Mosul—a spacious and level plain, with nothing more than a few undulating slopes, and without any trees. It was by nature well adapted for drawing up a numerous army, especially for the free manœuvres of cavalry, and the rush of scythed chariots; moreover, the Persian officers had been careful beforehand to level artificially such of the slopes as they thought inconvenient.[370] There seemed every thing in the ground to favor the operation both of the vast total, and the special forces, of Darius; who fancied that his defeat at Issus had been occasioned altogether by his having adventured himself in the narrow defiles of Kilikia—and that on open and level ground his superior numbers must be triumphant. He was even anxious that Alexander should come and attack him on the plain. Hence the undefended passage of the Tigris. [370] Arrian, iii. 8, 12. Καὶ γὰρ καὶ ὅσα ἀνώμαλα αὐτοῦ ἐς ἱππασίαν, ταῦτά τε ~ἐκ πολλοῦ~ οἱ Πέρσαι τοῖς τε ἅρμασιν ἐπελαύνειν εὐπετῆ πεποιήκεσαν καὶ τῇ ἵππῳ ἱππάσιμα.