after getting into stone's head quarters......the big fight....and his escape into his arena, you go to the left and fight a few more bad guys. when you get to the room at the end, you find a machine that keeps replicating walking skeletons. to shut it off, you grab all four of the spheres lining the outside wall and put them into the machine.
here's my question. when you enter the arena and kill everybody....up in one of the ramparts, there is a chest with another sphere in it.....what is it for? I have found no use for it as of yet.