Korea sits between three powerful cultures: China, the nomadic nations of Central Eurasia (particularly the Manchus), and Japan; and has felt the demographic influence of all three. The ancestral homeland of the Koreans includes both the Korean Peninsula and adjacent parts of modern Manchuria.
At the beginning of the study period, Korea was divided between three nations, the Koguryo in the Northeast, the Paekche in the southeast, and the Shilla in the southwest.
Fight and sixth centuries: Continuous conflict with China, which failed to conquer Koguryo under the Sui dynasty, but succeeded during the Tang dynasty by allying itself with Shilla. By the seventh century, only Shilla remained, governing the southern two thirds of the peninsula, while China and its allies governed the north. If your ancestors lived in northernmost Korea in CE 1000, their ancestors probably also lived in Northern China in 1 CE.
Tenth century: Turmoil in Manchuria resulted from the rise of the nomadic Khitan state. Refugees poured into Korea. If your ancestors lived in northern Korea in CE 1000, their ancestors probably also lived in Manchuria in 1 CE.
Eleventh century: It happened again, as the Jurchen (later called Manchus) organized their state. Refugees poured into Korea. If your ancestors lived in northern Korea in CE 1490, their ancestors probably also lived in Manchuria in 1000 CE.
1230s: Korea become part of the Mongol empire. Korea was backwater of their empire and were resisted by the Koreans, so probably little admixture of Mongol blood occurred. Still, it's possible. Mongol control faded during the thirteenth century.
Early 1600s: Manchus conquer, but do not invade: instead, Korea is made a vassal state of Manchu-controlled China, but has closed borders (the "Hermit Kingdom" period). Exceptionally little foreign gene flow.
1876: Japan forces commercial treaty with Korea. Korea is conquered and occupied by Japan from 1905-1945. If your family has Japanese ancestry, your family folklore will inform you.