I found a tunnel dug from one of the jail cells to what looks like the treasury on the main level, but I can't seem to find the trick for lowering the stepladder and getting into that treasury... and after having found Excalibur in the Queen's stash, I really wanna get in there! Anybody solve that puzzle? (the thing looks it might squash me if I trigger it from below, too!)
Also, has anyone been having trouble with getting into and out of Anton's shop? Seems like a bug to me.

Incorrect, I'm afraid. The tunnel you have found actually brings you up into the back of Ferro's armory. If you went back upstairs and raised his lift (the one you use to get to his stash with the armory key) then you'd see the ladder you are referring to. The only way to get anything out of the barred room with the chests is to use the deposit slips you find in two other places at the tube in the king's apartment.

I'm not sure what was up with all of the 'secret' tunnels you find in the cellar but none of them seemed to do any real good at all except for allowing you to move around the cellar without having to go back upstairs again.