To unlock the Auditore cape, you must find and collect all 100 Feathers scattered throughout the game. Once you collect all of the feathers, go back to the Villa and put them in the chest in Maria's room. After the short scene, you must equip it. To equip it, pause the game, go to your inventory, and finally outfits. Go to the Auditore cape, and hit to equip it. From there, you must visit each of the cities in the game to unlock this achievement. You must also visit the mountain region in the game, where you escort Leonardo in the carriage.