Assassin’s Creed is, of course, a fictional game series so all ‘historical’ events seen in it should be taken with a pinch of salt. However, it is not completely without historical merit and lots of the plots have a basis in truth.
Records do indicate that a guild of assassins was based in Masyaf Castle during the third crusade as seen in the first game. Although the many deaths of actual historical figures in the series are changed to assassination, they do occur at the right time and often the right place. A lot of credit must be given to the designers, as the surroundings and cities portrayed in the game, from the guards’ uniforms to the buildings the player ascends, are very close to how they would have been at the time.
The Knights Templar were a real military order, but their existence ended in 1312 after many of their members were burned at the stake, as seen in Assassin’s Creed Unity. The idea they secretly operate behind the scenes today can neither be proven nor disproved and exists solely in the realms of conspiracy.
Ultimately, Assassin’s Creed prevents a fictionalised version of history and should not be taken as fact by any means, but it does offer an intriguing basis on which to build your own historical knowledge of the people and events featured in it.
Masyaf Castle served as the stronghold of the Hashshashin sect, which the assassins in the series are based on