You'll find Antico's first Cluster on the northwestern lip of the Colosseum, just southwest of the viewpoint, and nearly on top of the landmark icon on the map (there's also a feather nearby, if you'd like to grab it while you're here).
From there, climb down and use the handholds to shimmy across until you're hanging right over the Cluster opening.
Drop down (holding B/Circle to grab onto the ledge as you fall), and climb in to access the next set of puzzles.
Aventino's a pretty big area, so we'll make the hunt simple: the Cluster's on the north-facing wall of the Borgia/Assassin tower on top of the hill.
This particular crack in the Animus's reality can be found on the north side of the acqueduct, near a Borgia/Assassin tower..
To get to it, though, you'll actually have to use that tower. Get over there and start climbing.
Soon you'll reach a platform that will put you within striking distance of the top of the acqueduct. Jump over there.
Once you've reached the acqueduct, stroll back over to where you saw the Cluster entrance, and get ready to drop down into it.
Once you've gained access to the Pope's castle, finding the final Cluster isn't too difficult. You'll need to make it to the top of the inner walls just south of the castle proper, where you should have a clear view of the place's facade.
The Cluster is embedded just over the castle's front gate; to reach it, just leap off the wall and grab that rose window, after which you should be able to easily climb down and inside.
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