User blog comment:The Deadliest Warrior/Alexander the Great vs. Genghis Khan: The Greatest Men on Earth Round 1/@comment-422690-20101127035709/@comment-422690-20101127152134

The ballista is not easily mobile, so while they will soften his men up they will be hard to bring to bear once he springs his retreat trap, which will have the added advantage of tiring Alexander's "fresh" troops, as well as the fact that Kahn's troops will close the distance faster than the men manning the Balistae will expect, leading to fewer volleys.