The other wheel is connected to a big axle which has studs sticking out from it. These hit the ends of 30 pound sledges which are mounted on pivots like seesaws. So there's a constant clanging din inside (-20 to listen checks, 50% chance a verbal spell fails), a hot area near the furnace (if you touch it, 2d6 heat damage), plenty of stuff to use as weapons, a huge rotating log that daring characters might fence upon (balance DC 15 every round, plus each attack and every time you get hit or fall to the ground below for a d6), and hammers that bang up and down on their own and could hit a careless combatant, or someone that falls of the axle (reflex save DC 13 or take 2d6 blunt damage if you end up under a hammer somehow).