Darius had been advised to send to the Semi´ramis Gate a body of those troops whom he could best spare: a thousand of them were cut to pieces. In a second sortie led by Zopyrus, two thousand Persians were slain; in a third, four thousand. This slaughter of seven thousand of his countrymen removed from the minds of the Babylonians all doubt of the truth of Zopyrus. The keys of the city were committed to his care, and the preparation for his treachery was now complete. During a concerted assault by the Persians, he opened the gates to Darius, who proceeded to take signal vengeance for the long defiance of his power. The reckless sacrifice of human life in this transaction shows how the habit of unlimited power had impaired the disposition of Darius, which was naturally merciful. =21.= To guard against future disturbances, Darius now endeavored to give a more thorough and efficient organization to the great empire, which Cyrus and Cambyses had built up. He divided the whole territory into twenty satrapies, or provinces, and imposed upon each a tribute according to its wealth. The native kings whom Cyrus had left upon their thrones were all swept away, and a Persian governor, usually connected by blood or marriage with the great king, was placed over each province. Order within and safety from without were secured by standing armies of Medes or Persians, posted at convenient stations throughout the empire. Royal roads were constructed and a system of couriers arranged, by which the court received constant and swift intelligence of all that occurred in the provinces. =22.= To prevent revolt, an elaborate system of checks was instituted, which left the satrap little power of independent action. In this earlier and stronger period of the consolidated empire, the satrap exercised only the civil government, the military being wielded by generals and commandants of garrisons, while, in Persia at least, the judicial power resided in judges appointed directly by the king. Beside these constitutional checks upon the satrap, there were in every province the “king’s eyes” and the “king’s ears,” in the persons of royal secretaries attached to his court, whose duty it was to communicate secretly and constantly with the sovereign, and to keep him informed of every occurrence within their respective districts.