/u/Talleyrayrand has previously answered Did professional assassins exist in medieval Europe? If so, where would they be trained? How would one become an assassin?

/u/mmjm123 has previously written about assassins/Templars during the Crusades.

For the most exhaustive game-by-game critique of Assassin's Creed, look at the index post for dissections by /u/VestigialLlama4 on /r/badhistory.

EDIT: There is a dissection post about AC: Odyssey as well.