Discover the Assassin's Tomb inside Santa Maria del Fiore (The Duomo).
Assassin’s Tombs are available to you once you complete the memory ‘Novella’s Secret’ in DNA Sequence 4 (refer to “Undertaker” for more information). To locate a tomb, search the map for a icon. If you have not synchronized the view points in that area yet, it will not be revealed to you. You must also have completed through a certain DNA Sequence for them to appear.
You can unlock this as early as DNA Sequence 4. This memory is called ‘Il Duomo’s Secret’ and is located along the north edge of The Duomo in Florence. Search for the skull “lever” along the wall at ground level behind a tree.
This tomb requires you to retrieve the seal from the sarcophagus located high up in the dome of The Duomo. This tomb is not timed nor are there any levers to pull to open other parts of the church. This is a straight open area jumping puzzle. You begin on the ground floor and the camera pans over to your start point. As you progress, ladders will fall so you don’t have to start again from the beginning, plus the game will register checkpoints in case you become desynchronized (if you die). Also, pigeons are discreetly placed along the path to assist you in finding your way. After making your way to the top of the dome, open the sarcophagus and retrieve the seal. When you exit the church, the achievement will unlock.
Once you have control, look for that stone throne or whatever it is that you saw during the cutscene beside the door. Climb it and use it to scale the wall. When you get up to where the murals are, shimmy over to the right and pull yourself up.
Jump to the beam to the north and from there, across two platforms. Jump to the next beam and move to the right as much as you can so you can jump to the next section. Move out so you can jump to the chandelier. Jump from one to the next, and then to the beam next to you. Jump to the south from the chandelier and you'll barely make it. A shortcut will be opened to you in case you fall off after this point.
Turn right (west) and go across the beam to the stone platform and climb it. Turn left at the top and jump from a beam to a swing