In the Foreman's dungeon. In the section with the bridge over the water if you go into the water and down you get into a cul-de-sac.
At the end is a collapsed walkway up far too high to jump to with a door marked on the map.
There are a set of posts sticking out the wall which used to hold the walkway which you can jump from one to the other on.
Getting to the top though is very difficult, looks possible though.
What's up there - how do you get up there?. If you stand inside the wall a bit you can see more dungeon extending and what looks like floaters in water.