Politician (Prime Minister); Commander-in-Chief. He became a high-level politician at 15 in succession of his father. In 642 A.D, he constructed Chol-li-jang-sung (1,000-ri (400km) Great Wall). In the same year, he seized a political power as a Dae-mak-ri-ji (Prime Minister) with King Bojang by overthrowing King Youngryu and his 180 fellow politicians who were opposed to him. In 645, Yeon, Gaesomun defeated Tang's hundreds of thousand troops sent by King Taizong several times.

The death of Yeon, Gaesomun brought Goguryeo(Koguryo) the fatal, internal disruption over the succession of political and military power, and this unfortunately led Goguryeo(Koguryo) be on the road to collapse.