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.