Discover the Assassin's Tomb inside the Rocca di Ravaldino fortress.
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 6. This memory is called ‘Ravaldino’s Secret’ and is located in the Rocca di Ravaldino fortress in southwest part of Forli. You can either climb one of the fortress towers and jump down or dive into the water and swim around the stronghold. There is a wooden platform around the back. Climb onto the platform and interact with the skull “lever” on the wall.
Throughout the fortress are timed switches you must activate that open gates that you must jump through before they close again. It is not at all difficult but you need to be quick to make it through. If not, backtrack and activate the switch again. Along the way, guards will be patrolling to impede your progress. Take them out by any means necessary. Follow through the fortress until you locate the burial chamber containing the sarcophagus. Open it and retrieve the seal. When you exit the fortress, the achievement will unlock.
While progressing through the game, make sure to constantly activate your Viewpoints and their Locations will be revealed. How you find them when you get inside is trial and error & a bit time consuming but in the end quite easy. Remember if you want "Altair's Costume," this is the only way to get it. Find the seal and take them back to the underground section at your Uncle's place in Monteriggioni. Enjoy..........................................................................