So, I'm feeling pretty stupid here, and don't know if I'm just missing something obvious. I've found the chamber with the -four five gates along the north wall and the big floor hole in the center. I can see/hear some of the chimes for the bastion beyond those gates, but can't for the life of me figure out how to access that area of the sewers. I've run the entire length and breadth of the tunnels, looking for every ceiling hole / grapple point, and I've found all the echoes in there.
Do you have to access that area of the sewers from a different entrance, or are there switches I've missed, or smashable walls?
Any help would be appreciated - my OCD won't let me move on from the sewers while there's a bastion to be solved, and I'm getting cold and wet down here.