The Siege of Kusong which occurred in 1231 was a decisive Goryeo victory against the Mongol Empire. After the Mongol army crossed the Yalu river, it quickly captured almost all of Goryeo's border defenses. The Mongol army however ran into stiff resistance both at Anju and the city of Kusong, commanded by General Park Seo. To take Kusong, Sartaq used a full array of siege weapons to bring down the city's defenses. Lines of catapults launched both boulders and molten metal at the cities walls.