How did the Mongols capture Beijing in 1215?

The Mongols captured Beijing, then known as Zhongdu, in 1215 after a prolonged campaign against the Jin dynasty.1

Initially, the Mongol armies began their assault in 1211, pillaging the countryside for several years before concentrating on the city in 1214. Because the city's fortifications were difficult to overcome, the Mongols initially agreed to a peace treaty and withdrew. However, when the Jin emperor subsequently moved his capital to Kaifeng, Genghis Khan viewed this as a breach of the armistice and renewed the attack.1

The Mongols, whose primary strength had been in cavalry warfare, initially lacked experience in siege craft. Their success in 1215 is attributed in part to the assistance of defectors from the Jin state, who provided the Mongols with essential knowledge of the territory and the conditions within the city. This victory effectively reduced the Jin to a small buffer state.12