The first concerted Mongol invasion of Jin occurred in 1211 and total conquest was not accomplished until 1234. In 1232 the Mongols besieged the Jin capital of Kaifeng and deployed gunpowder weapons along with other more conventional siege techniques such as building stockades, watchtowers, trenches, guardhouses, and forcing Chinese captives to haul supplies and fill moats. Jin scholar Liu Qi (劉祈) recounts in his memoir, "the attack against the city walls grew increasingly intense, and bombs rained down as enemy advanced." The Jin defenders also deployed gunpowder bombs as well as fire arrows ("huo jian" 火箭) launched using a type of early solid-propellant rocket. Of the bombs, Liu Qi writes, "From within the walls the defenders responded with a gunpowder bomb called the heaven-shaking-thunder bomb (震天雷). Whenever the troops encountered one, several men at a time would be turned into ashes." A more fact based and clear description of the bomb exists in the "History of Jin": "The heaven-shaking-thunder bomb is an iron vessel filled with gunpowder. When lighted with fire and shot off, it goes off like a crash of thunder that can be heard for a hundred li [thirty miles, burning an expanse of land more than half a mu a mu is a sixth of an acre, and the fire can even penetrate iron armor." A Ming official named He Mengchuan would encounter an old cache of these bombs three centuries later in the Xi'an area: "When I went on official business to Shaanxi Province, I saw on top of Xi'an's city walls an old stockpile of iron bombs. They were called 'heaven-shaking-thunder' bombs, and they were like an enclosed rice bowl with a hole at the top, just big enough to put your finger in. The troops said they hadn't been used for a very long time." Furthermore, he wrote, "When the powder goes off, the bomb rips open, and the iron pieces fly in all directions. That is how it is able to kill people and horses from far away."