I would reccomend:
1) watching the movie Labyrinth, with David Bowie as THE GOBLIN KING. The finale has a mock up of what you're discussing. Maybe the horror movie "Cube" too, for trap inspiration.
2) maybe use Demi-plane rules with Subjective Gravity, based off of whatever surface you're standing on as "down". Just remember, a 2D map is at best an INTERPRETATION of a 3D map... especially when you can walk on the bottom of the bridge you just crossed to get to the room "below" you. If you're planning combat, perhaps a couple prepared areas with only 2 walls/stairs to mess the orientation up with. I would NOT recomend flying creatures.
3) Escher never had Fly (which you've thought of), or Teleport... just saying. Magic is a key to unlock anything, if used correctly.
4) Beware a skill monkey trying to Acrobatics himself from ledge to ledge. If he misses, which way will he fall?
5) Whenever I try something new, I usually hand-wave a couple general rules and come up with the Very Specifics when I see the player reaction... nudge it one way if too easy, another if too dificult. Unless you have a group of rules lawyers, follow the KISS mentality: Keep It Simple, Stupid.
6) When you run this, find some good Escher pictures to hang on the walls to help them visualize the impossible geometry.
Just my 2cp.
;)
It will totally work. Go with it.
I wouldn't prohibit teleport, passwall, stone shaping, fly or acrobatics. Just assume a slow moving localized grav field around each PC that inverts unpredictably as they move toward other surfaces on a vertical plane so it turns into a nasty fall back to point of origin or to target location (50 50 chance) mid action. Basically, make such maneuvers result in chaotic trap effects ("you fly up... and smash into the ground for 3d6 dam). The "harder" they try, the more damage they take.
Um, of course using the same physics, ranged attacks on a chaotic vertical plane might have a 50% chance to rebound on the shooter/caster. Your call.
Escher's work also messed around with gradual transformation by inverting negative space into positive - ducks turning into fish, that sort of thing... lots of possibilities.
There has got to be a chaos creature involved somewhere, or perhaps an Order Domain cleric having a bad day, or just plain madness with planeshaping ability. Sounds very fun.
