If you haven't already seen the 'John Romero plays Doom' video that's been doing the rounds, you might want to check it out.
While it's interesting enough for Doom fans, with Romero discussing some of the changes that were made before the game was finished, it's particularly informative for level designers and game developers.
Presented by Double Fine, the footage sees Romero talk through the concepts behind the first level of the original Doom, including how the use of horseshoe-shaped rooms helps emphasise how non-linear the game was, and how they advanced the concept of secret areas started in Commander Keen and Wolfenstein.
"Here's kind of a golden rule of level design: finish the first level last," he added. "Because then you've learned everything about the game's level design, and you're going to put it all in the first level where you want people to get excited anyway."
Romero also discussed how diagonal corridors and zigzag paths helped show how advanced Doom was compared to Wolfenstein, and how mountains in China helped created one of the first outdoor skyboxes for a first-person shooter.
Last year, we interviewed Romero about the future of the FPS genre; you can find out what he had to say right here.
Check out the full video below: