Are there any airports with multi-level taxiways? There are many airports where roads (and pedestrian walkways) are routed underneath taxiways, runways, etc. But are there any airports where airplane taxiways go under/over each other?
I can't imagine there would be (for many logistical and sanity reasons), but in this world of unusual design and construction paradigms, I've learned to not say that something has never been done.  This could either be airports that are currently in use, or ones where they were used at one time, but are no longer active.
If you allow taxiway to also mean a surface where an airplane rolls but not under its own power, then there's the lower decks of an aircraft carrier, and Schätz's carousel hangars.  Note that even Schätz prefers scissors lifts to multilevel tarmac.
Multilevel tarmac would means ramps between levels, which is commonplace for surface vehicles but rare for airplanes, both in airframe design and in pilot training.