important part of Genghis Khan's warfare, especially in attacking fortified cities. The siege engines were not disassembled and carried by horses to be rebuilt at the site of the battle, as was the usual practice with European armies. Instead the Mongol horde would travel with skilled engineers who would build siege engines from scratch from materials on site. The engineers building the machines were recruited among captives, mostly from China and Persia, led by a Han general Guo Kan. When Mongols slaughtered the whole population from settlements that resisted or did not surrender, they often spared the engineers and other units, swiftly assimilating them into the Mongol armies. Engineers in Mongol service displayed a considerable degree of ingenuity and planning; during a siege of a fortified Chinese city, the defenders had taken care to remove all large rocks from the region to deny the Mongols an ammunition supply for their trebuchets, but the Mongol engineers resorted to cutting up logs which they soaked in water to make suitably heavy spheres. During the siege of the Assassins' fortress of Alamut the Mongols gathered large rocks from far and wide, piling them up in depots a day's journey from one another all the way to their siege lines so that a huge supply was available for the breaching batteries operating against the mighty citadel. The Mongols also scouted the hills around the city to find suitable higher ground on which to mount ballistas manned by northern Chinese engineers, allowing these to snipe into the interior of the fortress. The Mongols made effective use of the siege technologies developed by their subject peoples; Genghis Khan utilized the Chinese engineers and traction trebuchets he had gained from his victories over the Jurchens and Tanguts during his Khwarezmian campaign, while Kublai Khan later called upon Muslim engineers from his Ilkhanate cousins to build counterweight trebuchets that finally concluded the six year siege of Fancheng and Xiangyang. Kharash A commonly used Mongol tactic involved the use of the kharash. The Mongols would gather prisoners captured in previous battles, and would drive them forward in sieges and battles. These "shields" would often take the brunt of enemy arrows and crossbow-bolts, thus somewhat protecting the ethnically Mongol warriors.Commanders also used the kharash as assault units to breach walls. Strategy The Mongol battlefield tactics were a combination of masterful training with excellent communication and discipline in the chaos of