Yes glad you brought it up. I don’t do well with vertical and full 3D movement in games. See Wing Commander or Homeworld. Or Descent!
Doom couldn’t do rooms above other rooms. There are elevation differences but you are never standing “on top” of another room. That was one of the big engine limitations and it informed their level design. Another thing is that Romero and Carmack went for a “gameplay above all” approach. I don’t think they cared what the levels looked like. For them it was function over form. Romero recently (Edit: April 2016) made a new level for classic Doom and it’s exactly like that: no idea what it is or where you are, but fun to play.
Duke Nukem started out as a 2D platformer so when they made the transition to 3D I think it makes sense that the gameplay would be more vertical. We think of Duke3D as a FPS now, but I wonder if back then the designers were thinking “so how do we make a jump&run from a first-person view?”.
That’s also why I struggle with Duke3D more than Doom or Blood. I can’t for the life of me control the jetpack 😀
Blood plays a lot more like Doom. Shadow Warrior is more Duke.
