You can't climb everything... you need a ledge/something to hang onto in order to keep climbing, it probably looks like it from a specific angle, but there isn't anything there.
There's one view point in Acre during the 2nd assassination that will drive you insane... it took me like 5 minutes to figure out a way to climb the building (it's extremely tall and you have a ledge up until about halfway, but then you're stuck so you gotta start climbing to the SIDE until you get to the center of the building right above this one dude that keeps preacing to the public, and then climb down some and keep moving towards the center until you see a big ol' design on the wall which is finally able to be climbed... of course they made it this hard because there's a Templar waiting at the topbut still... it was rather annoying..) Oh and that building looked like it could be climbed further too, but the ledges just weren't there when I twisted the camera.
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