Cyrus' conquest was relatively humane compared to empires preceding his, such as the Babylonians and Assyrians. He allowed many areas of his empire a great deal of autonomy, rather than centrally controlling things. This distributed leadership allowed Persia to cover a greater geographical area than previous empires in that era and still be fairly stable, since most of its subjects were content to remain under Persian rule. Persian law was more codified and less arbitrary than previous empires; the king's seal declared approval of law which could only be reversed by further approved decrees -- not quite rule of law but going in that direction.
