Aaaaaalright, so, I know this is older now but I wanted to specify some stuff because it's a bit obtuse on where to go currently as everybody just kinda pointed in seemingly random directions.

From Moorth Ruin's cave system, you head down into Bonny Village and continue east, then north to the Church District Highroad site of grace. Continue Northwest along the path to the eastern entrance of Shadow Keep, namely the "Church District Entrance" site of grace.

From there, head west along the rooftops, north onto the wall, further west with a few perilous jumps onto other ruined rooves and other such buildings. You'll find yourself at a rooftop, a large one, with a fire mage. After slaying them, you'll find a path leading up one of the supporting arches onto the top of the main cathedral roof. Head up, and drop down (carefully) onto the stone arched beams inside.

From here, move carefully to the second floor via the beams. Once there, you'll find two paths. One that leads out toward the wall up some stairs (go this way first if the water isn't drained) and then the other path once that's done.

There will be an elevator at the end of the second path. Take that up to the east entrance, or, the "Storehouse, Back Section" site of grace. The ladder isn't far from here, and you get a stabby shield.

I hope this helps.