Which historical era is the primary setting of the first Assassin's Creed?

The primary historical setting of the first Assassin's Creed is the Third Crusade.

The game takes place in 1191, during the campaign traditionally associated with the Third Crusade. Altaïr Ibn-La'Ahad, a member of the Assassin Brotherhood, receives a series of targets across the Levant. His missions lead him through Jerusalem, Acre, and Damascus while he investigates a conspiracy involving the Templars.

The historical backdrop includes the struggle between Crusader states and forces led by figures such as Richard the Lionheart and Saladin. The game uses real cities and leaders, but its Assassin-Templar mythology and many personal events are fictionalized.

The First Crusade, the French Revolution, and the Italian Renaissance are all important eras elsewhere in the franchise. The Renaissance is the setting of Assassin's Creed II, the American Revolution frames Assassin's Creed III, and revolutionary France is central to Unity. The first game is specifically tied to the late twelfth century.