Wolfenstein 3D is widely known as the very first First Person Shooter that changed gaming forever. One of the most popular shareware games ever for 3 straight years in a row, Wolfenstein 3D puts you in the pants of BJ Blackowicz, an american prisoner of war in a Nazi dungeon where you fight off guards, SS, dogs, zombies, and more assorted nazis! It is a gem every FPS player should have in their games collection!
Abadonware for you to enjoy. Create mods for it if you like. Just google Wolfenstein modding.