Darius was several days in concentrating his vast armies upon this plain. He constructed encampments; he leveled the inequalities which would interfere with the movements of his great bodies of cavalry; he guarded the approaches, too, as much as possible. There is a little instrument used in war called a _caltrop_.[E] It consists of a small ball of iron, with several sharp points projecting from it one or two inches each way. If these instruments are thrown upon the ground at random, one of the points must necessarily be upward, and the horses that tread upon them are lamed and disabled at once. Darius caused caltrops to be scattered in the grass and along the roads, wherever the army of Alexander would be likely to approach his troops on the field of battle.