There are two small kegs of explosives in the game, both used at the beginning of the battle tower (one to clear the cave-in, and the other to open up a side passage shortly after that).
If that is the point you are at, there is a hidden storage room in the cell diagonally opposite the prisoner, to the left of a cell with a busted door (check the map and it should be obvious which wall section inside is movable)...
To get to the hidden storage room, there are a couple vertical wooden beams at the back of the cell, with a button on the left beam. After that there is another button (on the right IIRC) to remove another false wall.
The two small kegs of explosives inside are on a table, IIRC, and there is a bit of loot to collect.
I don't think there is a hidden room in Patriarch's cave, at least that you can get to.
What candle do you mean?