of Kaffa (now Feodosiya) during the siege of Caffa. It has been speculated that this operation may have been responsible for the advent of the Black Death in Europe. The Black Death is estimated to have killed 30%–60% of Europe's population. On the first night while laying siege to a city, the leader of the Mongol forces would lead from a white tent: if the city surrendered, all would be spared. On the second day, he would use a red tent: if the city surrendered, the men would all be killed, but the rest would be spared. On the third day, he would use a black tent: no quarter would be given. However, the Chinese were not completely defenseless, and from AD 1234 until 1279, the Southern Song Chinese held out against the enormous barrage of Mongol attacks. Much of this success in defense lay in the world's first use of gunpowder (i.e. with early flamethrowers, grenades, firearms, cannons, and land mines) to fight back against the Khitans, the Tanguts, the Jurchens, and then the Mongols. The Chinese of the Song period also discovered the explosive potential of packing hollowed cannonball shells with gunpowder. Written later around 1350 in the Huo Long Jing, this manuscript of Jiao Yu recorded an earlier Song-era cast-iron cannon known as the 'flying-cloud thunderclap eruptor' (fei yun pi-li pao). The manuscript stated that (Wade–Giles spelling): The shells (phao) are made of cast iron, as large as a bowl and shaped like a ball. Inside they contain half a pound of 'magic' gunpowder (shen huo). They are sent flying towards the enemy camp from an eruptor (mu phao); and when they get there a sound like a thunder-clap is heard, and flashes of light appear. If ten of these shells are fired successfully into the enemy camp, the whole place will be set ablaze... During the Ming Dynasty (AD 1368–1644), the Chinese were very concerned with city planning in regards to gunpowder warfare. The site for constructing the walls and the thickness of the walls in Beijing's Forbidden City were favoured by the Chinese Yongle Emperor (r. 1402–1424) because they were in pristine position to resist cannon volley and were built thick enough to withstand attacks from cannon fire. For more, see Technology of the Song dynasty. Age of gunpowder The introduction of gunpowder and the use of cannons brought about a new age in siege