Didn't it use lighting and shading effects and multiple alpha sprites? ISTR the magazine reviews discussing the fact that GS employed these techniques, seen for years on the PC, for the first time in an Amiga FPS.
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moto
Yeah, that's what I meant with transparent sprites, wasn't familiar with the technical term for them.

The 3D rendering is done with ray casting, just as in Wolfenstein 3D. It's just that the textures are waaaay prettier and the sprites have an alpha channel.