Which made it possible for mongol armies to conquer chinese walled cities?

Mongol spies and siege engines

The Mongols were an advanced military force but their strongest weapon came from the patience to gather information on an area before attacking it.

Mongol spies gathered information on walled cities looking for weak areas or people willing to aid the Mongols in their conquest. In addition they used weapons from all the areas they conquered adjusting the weapon choice based on the area. For walled cities, the Mongols used catapults and even burning of the surrounding area to isolate the cities.

The right answer would be a