The Koryeo dynasty (918~1392) was constantly war with outside settlers. The Kitan, Mongolian, and Yellow scarf thieves were invading from the north the Japanese constantly invaded the territory from the south In 1018, The Kitan had invaded the Koryeo, and General Kang, Gam Chan defeated them in the Guiju area. Also in 1107, General Kang built 9 castles. During the 13th century, the Mongolians from the North put Koryeo into a long history of war. In 1232, Koryeo defeated the Mongolians in the battle of Cheoin castle but eventually it was too difficult to defend the Mongolians in the 7th invasion. At the end of Koryeo Dynasty, Koryeo was suffering from invasion of yellow scarf thieves and Japanese invaders, Gunpowder was used after Choi, Mu Seon discovered the secret of making gunpowder.