The missions remain unlocked until you reach certain point in the story. (gamepur.com)
Although you play as Ezio for the majority of the game, you'll also take on the role of Altaïr and Desmond at certain points in the story. (ps3attitude.com)
At certain points in the story the main villains are able to twist the world in unique and interesting ways, and every time this happens I stop for a minute to take it in. (waytoomany.games)