To bind their kings with chains. Even royal captives were thus treated in the ancient world. Assyrian and Babylonian monarchs always represent their captives, even when kings, as fettered. Nebuchadnezzar "bound Zedekiah with fetters of brass" (2 Kings 25:7).
Parthia, and later Persia, and even Rome, followed the same practice. And their nobles with fetters of iron. On the monuments, cap-fives below the rank of kings are not often seen "fettered." Their arms, however, are frequently tied together with a cord, and they are fastened one to another by a stout rope.