The original Assassin's Creed is primarily set in 1191, during the Third Crusade.
Its historical story follows Altaïr Ibn-La'Ahad, an Assassin operating in the Levant. The game visits major cities including Jerusalem, Acre, and Damascus, placing its missions amid the conflict between Crusader and Muslim forces.
The year 1191 is important because it places the game near the end of the Third Crusade, which lasted from 1189 to 1192. Historical figures such as Richard the Lionheart and Saladin appear in the game's broader political setting, although the Assassin-focused plot is fictionalized.
The modern-day sections use a separate timeline involving Desmond Miles and the Animus. That framing story can make the chronology seem more complicated, but the historical gameplay sections are centered on 1191. The title's portrayal of real events and people is an imaginative interpretation rather than a literal historical record.