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After entering the Arsenal from its eastern gate, head northwest to find a smallish courtyard. The Fragment can be found under the eaves of this building:
Take a short jog to the northwest, and the next Fragment will be on a wooden platform suspended by a crane.
DID YOU LIKE THAT LAST WOODEN PLATFORM SUSPENDED BY A CRANE!? Well, hold on to your butts, because this one’s on a BIGGER platform, suspended by a BIGGER crane. You can find it dockside, southwest of Fragment 2.
Continue southwest from that crane, and hop into the under-construction boat skeleton. The Fragment’s just sitting there, waiting.
A little northwest of the last Fragment, you’ll find the next one sitting on a ledge above a bay window.
Head straight north from Fragment 5, and this one can clearly be seen just sort of floating above this alleyway:
Head west along the Arsenal’s wall, and the next Fragment is suspended over an arched gate out of the district. Swing across that pole to grab it.
Keep heading southwest along the wall. In the shadow of its next bend, you’ll be able to see Fragment 8 floating above a wooden ledge on the north face of a building.
Southeast of the last Fragment is a huge galleon sitting in the water, next to the top of a zipline. You’ll find Fragment 9 hiding from view at its stern.
Keep heading east across the water from the galleon, and you’ll find another big ship. This time, the Fragment’s exactly where you’d think it would be, which is to say at the top of one of its masts (the one on the western side, specifically). Climb up there.
If you’re not sure how to get all the way up, remember that you can climb up the rope rigging around the crow’s nest.
From the ship, get back onto dry land by heading east. Arsenal’s final Fragment is just across from the dock, in the rafters of this barn-like building: