The opening of the first Assassin's Creed primarily depicts the year 1191.
The historical sections follow Altaïr Ibn-La'Ahad during the Third Crusade. The game begins with Altaïr and fellow Assassins operating in the Holy Land, and its principal cities include Jerusalem, Acre, and Damascus. These chapters present a fictionalized conflict between the Assassins and Templars against the backdrop of the Crusades.
The year 1191 places the story during the reign of Richard I of England, often called Richard the Lionheart. The game also includes historical figures such as Robert de Sablé and William of Montferrat, though the central plot and many characters are fictionalized for the series.
The dates in other Assassin's Creed stories can cause confusion. 1099 is associated with the First Crusade and the capture of Jerusalem, while 1453 is the year Constantinople fell to the Ottoman Empire. The later game Revelations includes events in 1511, but the original game's main historical opening is set in 1191.