When you make your way to the outside of the fortress for the first time there is an area up left with a couple of corpses waiting to be pillaged look to your right there should be a broken wall that looks like a fall to your death if you edge up and move
the camera around you can see a platform and a bonfire below you.
Yes, you have to drop off the side of the fortress to get to it though so it isn't overly obvious. It's near where you go out onto the roof up the first small set of stairs and there is a break in one of the walls you can drop down to it.
There is a video showing you where it exactly is if you need a visual reference.