The original Assassin's Creed uses the Third Crusade as the historical setting for Altaïr's story in 1191.
The game follows Altaïr Ibn-La'Ahad, an Assassin operating in cities including Masyaf, Jerusalem, Acre, and Damascus. These locations place the narrative during the conflict between European Crusaders and Muslim forces in the Holy Land.
The Third Crusade lasted from 1189 to 1192 and was associated with figures such as Richard I of England and Saladin. Assassin's Creed dramatizes real places and leaders, but its secret Assassin-Templar conflict is fictionalized.
A common mix-up is confusing the game's setting with the First Crusade, which began in 1096. The game's date, 1191, falls within the Third Crusade.