I played two other previous games and I must confess this is one of the most graphical games I played. The graphics is astonishing and Istanbul is really charming. You can roam there freely and liberate it from the Templars in some fierce battles. Or you can follow the main lead and discover the secrets in Altair's library. You can upgrade your armour or you can buy new ones. You can do almost everything: swim, jump, assassinate from the air, use a parachute, control a carriage.
I won't spoil the story because it's really interesting and the secrets can be found out only at the end.