We had our first RtL session last night, and this dungeon came up. We were unclear on whether the monsters were able to open the prison cell doors. The FAQ says to consider them undiscovered areas for purposes of spawning, but that doesn't necessarily mean the monsters can't open the doors.
We played it as though they were not allowed to open them. And I, the Overlord, got my butt beat in there. I only managed to kill one villager (got another down to 1, but couldn't seal the deal).