Getting near the prisoner's cell should trigger a conversation, where he tells you about a hidden storage room in the cell with the wooden beams (in the cell diagonally opposite from the prisoner, the left beam should have a button to remove a false wall).
Before getting to the prisoner, though, there should have been a cutscene with the undead Sassan, where the passage to the elevator was blocked by a cave-in, requiring you to get into the storage room in order to clear the way.
One other person ran into this problem, but after finishing off the remaining bandits on the path outside, it worked properly (
Xbox360 US: Complex Beneath the Battle Tower).
If that doesn't help, do you have a recent save before entering the battle tower? If so, I'd reload to be on the safe side, and head back to Broken Valley (otherwise do so in yout latest save). If there was just a glitch loading the battle tower, maybe when you return the initial cutscene will get triggered and everything else will work properly.
Hopefully it is just the game waiting for the bandits to get wiped out, though.
