I am trying to get to the assassins tomb on the far right side of venice inside of that large fortress type area where you originally flew to with Leondardo's aircraft... After that mission, everything is sealed back off and I don't even see a way to get to the tomb. How do you do it?
go around the back to the red building (it's connected to the building you are trying to get into), climb up to the top and move to the front of the building and look for one of the balconies that has the mark on the map for the tomb, you might be attacked right when you find it.
Hope is the first step on the road to disappointment - Librarian MLG is a joke, end of story.
Unfortunately, that video doesn't show how to get to the tomb... Basically there are two sides to that massive building, and it looks like you are unable to get from one side to the other (when on the roof) because there are fences stopping you. The only way you got inside before was with Leonardo's flying machine...
The assassin tomb is NOT in the restricted building.
After you complete with Da Vinci's flying machine you no longer need to access it.
The assassin's tomb is right next to the restricted building. Basically it's the red building you use to climb near the restricted building. You know where the dome is on red building. There's a little drop down area - that is the entrance to the assassin's tomb.