Sai is a city in the state of Qin.
Sai is located at the end of the Bu Pass, the southern path that leads to the capital, Kanyou. It is also in front of the Wei river and Kanyou. Thus, if any invading army were to march onto Kanyou, capturing Sai is necessary lest they wish to be ambushed while trying to cross the river.
Sai is a small city, which is protected by medium sized walls. During the Coaliton War, it was populated by a population of about 30,000 with half being women, children, infirm and elderly as the men who were fit were sent to reinforce Kankoku Pass. It's four gates are durable but the walls are low, enabling any enemy army to scale the walls with just their ladders. After the Battle of Sai, it's population was reduced to half its original size before being repopulated after the battle.
The city of Sai was used by the state of Qin as an armory, that provided the armies of the southern pass with weapons and armor.
Upon the news of the Coalition Invasion, Qin gathered every man able to fight and sent them to Kankoku Pass. Sai answered the call and sent all the men to join the army, leaving behind only the unfit for duty, women, children and elderly.
In a desperate move to stop the advance of the Ri Boku Army, king Ei Sei gathered his army and moved to Sai to make there his last stand. Upon arrival, he met up with Shin and his unit, who incidentally stopped there to regain strength after being chased by Ri Boku's soldiers.
Sei, noticing that all he had was old people, women and citizens who were out of their prime, he decided to reveal himself in order to raise their morale. He went on to give a moving speech which touched everyone's hearts and convinced them to fight to their last breath in order to protect their kingdom.
After a fierce battle which lasted for 7 days (see Battle of Sai) the Qin army with the great contribution of Sai's citizens managed to protect the city and repel the Zhao army.