Interesting article. I played with making Doom levels back in the day and it wasn't exactly easy. The tools (DEU, DETH, etc.) were generally clunky DOS apps that resembled very simplified CAD programs. Documentation was non-existent or spotty at best, so you really had to spend a lot of time figuring out how to do simple things. It would easily take you months to ramp up and start building levels that were more complex than two rooms with a door (and even that was pretty complex to figure out the right line defs, sectors, actions, etc. to just make a door work!). This was before the web took off so you couldn't just Google for a Stackoverflow answer when you got stuck!

Duke Nukem 3D and the Build engine were pretty interesting for level creation. The game engine included the editor so you could actually drop into a mode where you walk through the level and directly modify textures, floor/ceiling height, etc. Was a very cool experience at the time.