Did you check all the cells in the room where you got the cutscene with the undead Sassan? If you release a prisoner there he will tell you about a hidden passage in one of the other cells...
The cell is diagonally opposite the one with the prisoner, to the left IIRC of a cell with a bashed in door. Check the map, and you should be able to tell which wall is false.

In more detail
In the cell there is a button to open the false wall, on the right IIRC of two vertical wooden beams. In the next room there is another false wall, and another button on a beam, this time on the left, IIRC.

Once in the second part of the hidden storage area
you can find 2 small kegs of explosives, on a table before you reach a chest at the back of the room. One keg can be used to clear the cave-in, while the other can be used to open a small side passage shortly after there.