I recall some systems (linux specially) having issues with texturing there, showing black areas in front of the citadel.
Although, that meant (I think) non lighted textured (cached lighting not found) but I guess you could walk on them, normally.
Seems that, what you found, is an actual hole in the map. If it isn't the hole where the water falls...

Try approaching that spot once more, and use the /pos command so the exact location can be stated and someone might take a look there.