The Thirteenth Floor is one of my favourite movies of all time. Truly mind-mangling, it is what The Matrix wishes it could be but isn't.
In Derpy's case, where is the top of the helter-skelter? At what level is she and where is the real world... if there is even such a thing? Is she a person trapped in a simulation... or is she a simulated person trapped in a simulation created by the simulations in the simulation?
It's questions like that which make The Thirteenth Floor not the sort of film you can watch without having drunk coffee beforehand