Been exploring the turtle shell dungeon, beat the first boss and got the floaties, found a green gem that I fit into the skull's right eye (Well, right from my perspective, technically it's the skull's "left", but that's irrelevant; I think).

I can't seen to find the other gem, and I have a sneaking suspicion that it's somewhere behind the barred door in the room to the left of the skull room. But after what felt like an age of poking and prodding the room with all my tools, I can't seem to figure out the mechanism for the door.

I don't want all the details for how to get the gem or the boss (I'd still like to figure out as much of this game as I can), but I at least need a solution for opening the barred door; because right now, I'm wasting my time and giving myself a headache. X_X