How do you play as Altair in assassins creed revalations

Well you have to get the keys to Altairs Library from templars but in order to do so you must go constinopole and help out Ysurf and his group of assassins then while doing that you will search for the keys their is six in total and when you get the key you get one key to open the door to the library and then relive key moments in his life