In 1992: Id Software released Wolfenstein 3-D. This may not have been technically first 3d first-person shooter. However, for most of us gamers this was our first exposure to a first-person shooter with full 360 degree smooth rotation in the space.
Wolfenstein 3-D introduced gamers (actually millions of them) to a fully immersive environment.
Gorebagg and friends quickly found themselves playing Wolfenstein and more importantly making custom maps for Wolfenstein.
Gorebagg would make a new set of maps almost daily for distribution to a close-knit group of friends equipped with the core engine.
By today's standards the resources were limited. But, keep in mind the day before Wolfenstein was released the resources and ability to make levels like this was zero. A definite step up.