Atop a ruined stand of columns out behind the Palazzo Senatorio. To get up there, climb the stairs up to the Palazzo and leap across.
The next flag is hiding above the central arch of the Basilica di Massenzio.
Once you're up there, look to the northeast, and you'll see the flag hiding amongst the bushes.
Just west of the Arco di Tito, and northwest of the Arco di Costantino, is a sprawling ruin. Clamber on in to find the next flag on top of this column:
This one's pretty easy to spot; it's on top of the Arco di Costantino, just west of the Colosseum.
A little southwest of Flag 3, you'll find this collection of ruined columns next to part of a broken acqueduct. If you're having any trouble finding the flag, try taking on the nearby Thief race; your course will take you right through it.
The next flag is on top of a set of ruined columns at Colle Palatino, overlooking the Circo Massimo.
Antico Flags 7-11
If you're entering Aventino from the east and look to your right, you'll see this flag just sitting on a low column amid some ruins. Run up and grab that sucker.
Heading south and up Aventino's hill, you'll find a Borgia tower. Poke around the buildings just south of it and you'll see a column with your next prize.
Northwest of the Piramide Cestia (and northeast of the Thieves' Guild, which is in the "undiscovered" part of our map below), you'll discover a ruin with yet another column, on top of which rests your next flag.
Travel due northwest from Flag 9, and you'll find another brace of columns next to a pigeon coop. Clamber up the nearby columns in order to get up there.
Situated just south of an acqueduct, this flag's easy to spot, but seemingly impossible to get to. If you want to grab it, you'll have to find a way up to the acqueduct.
Head west across the valley and onto the next hill. Your goal is the western end of the acqueduct.
Once you've reached it, just climb up the western face...
... and run all the way back to where you spotted the flag. It should now be within easy jumping distance.
Antico Flags 12-18
The next flag's sitting on the southeastern lip of the Colosseum's ruined outer wall. Just climb up there and it's yours.
Due east of the Colosseum, and almost directly west of the Terme di Traiano, there's an archway leading to a set of ruins. The next flag's on a column right next to it.
Head east of Flag 13's location, and you'll see the next flag on top of another ruined column stand just south of the Terme di Traiano landmark icon.
Don't leave the Terme just yet; there are a couple more flags to grab. The next one's on top of yet another set of columns, this time just northeast of the Terme's landmark icon.
Head to the east end of the Terme di Traiano, and you'll see this flag atop one more set of ruined columns. Unlike the others, though, this one's tricky, as there's no immediately obvious way up there.
There might be an easier way up there than the one we're about to detail, but if there is we weren't able to figure it out. Before you begin, make sure you've bought the climbing glove from Leonardo, and earned the parachutes by destroying all of his war machines. Assuming you're inventory's up to speed, make a run for this archway southwest of the flag, and climb up there.
Once you've climbed it, look to the east and you'll see this curving wall of arches. Jump over there.
Head around to the far side of the curve, turn west and jump across the two pillar platforms in front of you, then turn and hop over to the pillar that's visible on the right of this image:
Once you've reached that platform, seemingly all you have to do is jump down to the next platform - which is just out of your jumping range. To reach it, jump, deploy your parachute, and then immediately release it. You should land safely on top of the column, at which point the next flag is within easy reach.
After that last flag, this one's simplicity should come as a relief. Look for it on another ruined column stand, this time next to the gate at the eastern edge of Antico.
From Flag 17, take a long jog west until you're roughly southeast of the Colosseum, near the top of the more northern "locked" area that doesn't become available until later in the game. The next flag's on top of another set of columns slightly west of the road.
Next page: Flags in the "locked" areas of Antico, the final three feathers, and flags in the Vaticano district!
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