The first Assassin's Creed game is primarily set in 1191.
The story takes place during the Third Crusade, a period of conflict in the medieval Levant. Its major locations include Masyaf, Jerusalem, Acre, and Damascus, where Altaïr Ibn-La'Ahad carries out missions for the Assassin Brotherhood.
The year is important because it places the game during the campaign associated with the Third Crusade, which began in 1189 and ended in 1192. The historical characters and political tensions are inspired by real events, although the secret conflict between Assassins and Templars is fictionalized for the series.
A common mix-up is the date 1099, the year of the First Crusade's capture of Jerusalem. Assassin's Creed instead uses the later Third Crusade as its principal historical backdrop.