How it's made. The channel's floor is a designed profile, and the profile is the throttle: a hump bleeds speed before a turn, the dip after it gives speed back. It is built to an energy budget - at every point the marble has head to spare, at every crest it is slow enough to stay on the track, and the floor is never steep enough to be a drop - and that budget is checked by simulating the roll. The towers abut and the channel is carved through them from above, so the whole castle is one piece with nothing overhanging. Takes an ordinary 16 mm marble.