Before Doom was Wolfenstein 3D, ID Software’s first 3D title and the first available first person shooter on the PC. That title was actually inspired by an old Apple ][ game called Castle Wolfenstein, which was a third person shooter with an overhead view written by the short lived but major player MUSE Software. MUSE was also the first company to produce a “copy protected” diskette for their software distributon. https://en.wikipedia.org/wiki/Muse_Software