If you are like me, you have spent a considerable amount of time with the map simply going to the flags and claiming them with whatever crazy jumping is necessary. The one flag that I had difficulties reaching, however, is the one far south in Antico, in the previously restricted section to the west of Terme di Caracala. The area seems to be blocked off by some high rock walls except for at a broken bridge which somehow needs to be jumped. That bridge cannot be jumped across, even with the standard jump and then grasp (B) or jumping and deploying a parachute immediately afterwards. The key, as I eventually found out, is to grab a horse (you can whistle for one with Y) and gallop across the gap. Grabbing the flag after that is nothing special. Hope that this helps if you're like me and tearing your hair out.
Edit: Thanks to sarahhillery for pointing out that armored guard horses WILL NOT work for jumping the gap. Get an ordinary horse. If you need a new one, whistle for one with Y.
Edit: Thanks to ozmosis82 for pointing out that it is in fact possible to jump from the rail of the bridge and then immediately deploy your parachute, thus just barely making the leap safely. It might still be easier to go for it with a horse but that is definitely a welcome update!