However, the Jin returned soon after with enough siege machinery to scale Kaifeng's layer of walls defended by 48,000 Song troops. The Jin used siege towers taller than Kaifeng's walls in order to lob incendiary bombs into the city. The besieged city was captured by the Jurchens in less than two months. Three thousand members of the Emperor's court were taken as captives, including Qinzong and many of his relatives, craftsmen, engineers, goldsmiths, silversmiths, blacksmiths, weavers and tailors, Daoist priests, and female entertainers to label some. The mechanical clock tower designed by Su Song and erected in 1094 was also disassembled and its components carted back north, along with many clock-making millwrights and maintenance engineers that would cause a setback in technical advances for the Song court. According to the contemporary Xia Shaozeng, other war booty included 20,000 fire arrows that were handed over to the Jurchens upon taking the city.