I'm missing something obvious... I have spent hours swimming around in the sunken city and I cannot find anywhere to use my glowstones (I have two of them). Is the circular room the room that turns? I've gone down the sewer grate, I went to the library, I talked to the oracle, I went east out of the library, wound around and fought a bunch of crabs. I pushed the lever to exit the crab room to the south... I went into the circular room (just pushed a lever.. didn't see a lock). Went out the only door to the west, wandered around over there until I ended back up in the circular room. I've done this about a dozen times and still don't see where to use the glowstones. Some kind soul, please have pity on me...

A glowstone will fit into the circle room lock (east) and open a door across the room. Wait for it to rotate and get off there. The other glowstone works in another circle room lock (south side). You'll want to recover the east glowstone then - it's used later. And there's a third glowstone in the suspended trolley room.

Hope this helps.

I haven't found any locks... There is a lever just before I go in the east door of the circular room, but nothing has happened when I tried using the glowstone on the lever.