As you're looking at the map, you need to go to the left (west) and then south until you can go south no more, then east. With so many different steps & stairways, it can be a bit of a maze at first, but if you systematically go everywhere that you can on the map on the west side of the Waterway, you should be able to get there.

There's even a save crystal right before you get the mark, if memory serves.