645 AD. Emperor Taizong invades Goguryeo with his massive Tang army. His initial target is the fortress at Ansi. After defeating a major attack by the Goguryeon army he besieges Ansi. The odds are massively stacked in Taizong's favour: opposing his 200,000-strong army is a mere 5,000 force at Ansi. It is lead by Yang Manchun, a man regarded as a traitor by the Goguryeon leader.