XThe Amory Wars' solution is spot on but if you want to do it quicker without running around trying to find a lift somewhere try this:
Load Mission: The Ezio Auditore Affair, memory 6 of the Da Vinci Disappearance Missions
Once you enter the Castel, walk around to the north east corner of the green rectangle on your map (the area you must search to find the painting) and one guard in particular is standing right under a lift. Just run into it and it'll pop him on the head....done, and you won't even be detected.