This achievement is not too hard to get, if you are prepared for the climb to it.
First off, a few notes:
- This is found in Florence, the first Italian (past) city you experience
- The Giotto's Campanile is in the NE of the map you begin with, you will find it on your map marked as an Eagle along with the other vantage points
- There is one red guard to the east of the tower, try not to use any aggressive (Right Trigger) actions near him to avoid being chased down
The real trick here is that to get to the top you need to use both the large cathedral like building and the tower to the South of it; both are clearly visible on the map upon exploring the area.
Climb up the South face of the Cathedral's wall, opposite the tower. Then work your way to the West (left) until you reach the corner, which will be an outcropping, climp it up to the first roof.
Turn around and look for the black, tall, rectangular window on the opposing tower; jump across to this window with RT + A.
From here hold RT and proceed upwards, about two levels up there is a fully broken out window on the East side with a chest inside with some florins. After getting this exit the window (B to drop down), shuffle to the North side of the tower and continue upwards, you will reach the top.
Upon reaching the top, synchronize first (Y), then do a leap of faith from the wooden outcropping (RT + A + Direction).
Enjoy your achievement.
Please comment or message me anything you'd like improved on this guide before/after negatively rating.