In those strange aeons before Starbucks and iPods made the white, bitten apple ubiquitous, those of us who held fast to the rainbow fruit lived in an alternate gaming world, two years behind everyone else and echoing their statements in similar, if slightly different languages. Deprived of the vast torrent of Doom clones sweeping PCdom, we contented ourselves with the visually dark, narratively dense and occasionally surreal shooter Marathon, from a Bungie Studios that consisted of a dozen or so dudes and bug reports logged on a mountain of pizza boxes.
One of the most interesting levels was the result of Marathon's not-actually-3d software, which extrapolated 2d geometry into the illusion of 3d space. So, while you couldn't make a simple ramp, you could force players to battle each other in two overlapping, intersecting grids which occupied the same 'space.'
Since then (and Myst, et al), I've been somewhat obsessed with the exploration of spaces which defied normal human experience, whether they're just regular labyrinths of interconnected 5d cubes or dungeons with multiple concurrent timelines overlapping. As an introduction, I submit a map of the Marathon level drawn for insertion into friendly a dungeon near you. All you need is a teleport error or a maze spell trap to dump your PCs into it. Because of the weird sightlines, I recommend filling it with ranged attacks, flying monsters, gelatinous cubes or mobile area effects to chase PCs around corners.