This is how the back (top) of the bridge looks before adding a horizontal layer of flat-ish stones
After all the voussoirs are firmly set in place,the back of a bridge (the area that is walked on) should be thoroughly pinned and then built over with random shaped filler stones. They don't have to be really good stones.
All of them can to be laid on their flat to within 6 inches of the finished height of the walking area bridge. The stones should be fitted together carefully so they don't rock.
Then the parapet or border wall of copes are laid the way you would standard vertical coping stones on a dry stone wall.
A layer of gravel is then added in wheelbarrow loads over the back of the bridge.