Here is an extremely interesting overview/review of the original Doom Engine source code. This page discusses the engine in much detail, discussing map design, view points, wall sructure, etc. It’s a really interesting read if you’re into learning more about older 3D video game engines. via Doom Engine source code review (fabiensanglard.net)

Additional Resources

• Original Doom source code (source code was released in 1997)
• Chocolate Doom (a popular and excellent source port)