There isn't one. The castle, as it's portrayed in the movie doesn't exist. It is absolutely impossible for the elevator to be where it is in the lab, and for the hole in the wall to be where it is. You'll also note that there are no doors into the lab. So how do the Transylvanians get there before Brad and Janet? Perhaps they all made a quick trip in the elevator, perhaps they all teleported up there, or perhaps they used Riff and Magenta's secret entrance that was originally supposed to be there. This also was supposed to be the way in which Dr. Scott entered the lab, but nobody really noticed that they forgot to remedy this until it was being filmed. Richard asked Brian how Dr. Scott was going to enter, and Brain answered, as a joke, "We'll just push him through the wall." Also, is that a rope I see pulling away the castle backdrop as it takes off?