Francesco Salviati is very persuasive.
http://www.youtube.com/watch?v=8ZZUmezCI2k
After you leave the area, the gate to the compound is no longer modeled in the game. The developers forgot to put it back in place when you return. This mission is loosely related to "There is no spoon"-type missions:
http://www.thehiddenblade.com/dante-moro-and-silvio-barbarigo-assassinat...
I did this mission today and I did it just like that! Except slightly messier - just slightly, mind you! Thanks Ian - that made it a lot of fun. 
so glad you tried it out and found it fun! ive known about this bug for a few months. (it took me a while to figure out a way to make it interesting enough for a standalone vid.) as you know, i usually quit the memory to replay the assassination mission, but sometimes i want to progress the game. in those cases, i almost always use this trick. its fast, and its funny
There's usually memory walls to prevent things like that from happening in the game, but they forgot to put one in this mission.
Gamertag:THBmemberAurel
There's a time to stalk and a time to stab.