The attack was designed as a show of overwhelming strength for which the audience was not the already conquered people of Bukhara, but the still distant army and people of Samarkand, the next city on his march. The Mongol invaders rolled up their newly constructed siege engines--catapults, trebuchets, and mangonels that hurled not only stones and fire, as besieging armies had done for centuries, but also pots of burning liquids, exploding devices, and incendiary materials. They maneuvered immense crossbows mounted on wheels, and great teams of men pushed in portable towers with retractable ladders from which they could shoot down at the defenders of the walls. At the same time that they attacked through the air, miners went to work digging into the earth to undermine the walls by sapping. During this awesome display of technological prowess in the air, on the land, and beneath the earth, Genghis Khan heightened the psychological tension by forcing prisoners, in some cases the captured comrades of the men still in the citadel, to rush forward until their bodies filled the moat and made live ramparts over which other prisoners pushed the engines of war.