Know your enemy.

This video is an interesting behind-the-scenes look into the capabilities and senses of each different enemy in the 1993 videogame DooM.

• Bonus content and nostalgia:

A look at where consumer available FPS began with Maze Wars+ (1987)

Maze Wars+ was the first experience of a networked-multiplayer-graphical-game for many people, and completely mindblowing for the era.

• For a current quality FPS experience, see Cube 2: Sauerbraten