Meanwhile, another Mongol force under Chagatai and Ogedei descended from either the Altai Mountains to the north or the Dzungarian Gate and immediately started laying siege to the border city of Otrar. Rashid Al-Din stated that Otrar had a garrison of 20,000 while Juvayni claimed 60,000 (horsemen and militia), though like the army figures given in most medieval chronicles, these numbers should be treated with caution and are probably exaggerated by an order of magnitude considering the size of the city. Genghis, who had marched through the Altai mountains, kept his main force further back near the mountain ranges, and stayed out of contact. Frank McLynn argues that this disposition can only be explained as Genghis laying a trap for the Shah. Because Shah decided to march his army up from Samarkand to attack the besiegers of Otrar, Genghis could then rapidly encircle the Shah's army from the rear. However, the Shah dodged the trap, and Genghis had to change plans.