Discover the Assassin's Tomb inside the Basilica di San Marco.
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 8. This memory is called ‘San Marco's Secret’ and is located in the restricted area in the San Marco district of Venice. You must first climb the Basilica di San Marco to reach the tomb entrance. It is easiest to use the same ascent that you made in the second memory in DNA Sequence 8 called ‘If at First You Don’t Succeed’. Run along the roof and climb down a ladder and the skull “lever” is against the wall.
For this tomb, your goal is to activate four switches in the four sections of the church before time runs out. When you begin, walk towards the sarcophagus on the ground floor and activate the switch. This will reveal four floor places in the center of the church. Stepping on each will start a timed jumping puzzle. They are not that difficult and the camera shows you the path you need to take. If you think you’ll run out of time, stepping on the floor plate again resets the timer.
Once you’ve activated the four switches, the center of the floor will open up revealing steps. Follow the steps to uncover the tomb. Open the sarcophagus and retrieve the seal. When you exit the church, the achievement will unlock.
this one took a little more time than the rest because of the "jump in a slightly random direction" that happens occasionally... one or two of the routes are a bit of a pain, but you have plenty of time to get to them as long as you get used to the way he moves
This is probably the easiest I have done so far. All takes place in 1 area and its rather easy to activate the levers. I only had to repeat them 2 times to get it - The first time is to find the path and get to it and the second time is to pull it