[2] However, as Tang gained strength, in 631, it sent a small force to destroy a monument to Goguryeo's victory over the Sui.
In response, Goguryeo built the Cheolli Jangseong defensive wall along the western border, a 15-year project begun in 631 under the supervision of Yeon Gaesomun.
[2][1] During this time, Goguryeo continued its battles to recover lost territory from the southern Korean kingdom Silla.
They planned first to kill Yeon Gaesomun, whose power and influence were rapidly overtaking the throne's.
[4] The young man eventually discovered the plot,[4] and immediately went to Pyongyang to kill the plotters, including the king.