If you invade the Mercenary Guild early in the game, before your party can take the kind of damage that those Masters of the Sword can dish out, that jail cell can be an invaluable asset. Its secret doors lead to a room full of baddies. You can let them follow you back through the passage, beat them to the door, and close it behind you.While they're all stuck in the jail cell, you can get just barely into melee range (but just outside of their reach -- your reach is longer) and whittle them down with no reprisals. Once you get them all, you can even reverse this tactic by leading over their friends from the other side of the building, and placing yourself in the jail cell, but this will require more patience.
BV