As Yahtzee said, this could have been a much shorter final bit of story that just never made it to the end of Assassins' creed 2 so instead they adapted it into an entire game since they were making a multiplayer game anyways and figured that having a full single player part with it would be awesome. Rome could have just been a final city with one last chapter to tie things up but instead would up getting cut then edited into a whole story.