The crab's claw points steadily towards the faint depression in the cell wall as you move towards it to take a closer look. The stone here appears darker, the mortar brittle and ancient.
You glance back towards the cell door with your one good eye, making sure no one is around to overhear, then give the darker segment of cell wall a mighty shove. As the mortar gives way unexpectedly, the large chunk of stone you shoved falls through to the other side with a muffled thunk. You sink arm-deep into the wall, unprepared for the lack of resistance.
Feeling around the edges of the newborn hole in the wall, you push a little more gently on the piece directly below, maybe two feet off the ground. It grinds the stone below it as it moves, but move it does, until you manage to wriggle it all the way through. It drops, hitting its sibling as it makes its way towards the ground on the other side of the wall. The lack of any kind of resonance through the gap in the wall makes you believe that the floor must be packed earth of some kind.
A moment later you've managed to shove enough of the rotting wall through to make your way across to the other side. You consider for a moment, then move through the opening, feeling slowly with one foot and then the other. Avoiding the rubble of the wall, you put a foot down onto what feels like a dirt floor. The air is musty and thick around you. You appear to be in the middle of a corridor between or behind the walls of your captors' cells.
To the left, you feel a mild breeze; to the right, you hear the faint sound of running water. Rumour has it, an underground river flows beneath this fortress.
Behind you, the crab appears to be following. It sits at the foot of the last of the crumbling wall, one eye stalk aimed curiously in your direction. You pick the small crab up gingerly and place it in the front pocket of your shirt — it got you this far, so might as well bring it along for the ride. The crab seems content, sitting silently, close to your rapidly beating heart.
Your path is unclear.