The Fortress is set in 1636, when the Chinese Qing Dynasty has invaded Korea with 150,000 troops. The Korean court flees the capital to take shelter in the Namhan Fortress, located in treacherous mountains outside the city. The Qing army, led by General Yong Gol Dae, quickly surrounds the fortress, leaving the king and the court stranded in a prolonged standoff. For a truce, the Qing general makes a humiliating proposal that they send the Crown Prince out as hostage. Unable to move forward or fight the enemy they are outnumbered by, the people of the fortress start dying from cold and hunger. The king now faces an impossible choice between protecting his people and standing up to those looking to destroy them.
