It's hard to criticize the single most influential FPS of all time without sounding nitpicky or contrarian, but the truth is that while Doom is masterful in many ways, it very apparently left room for improvement, as evidenced by Ultimate Doom's additional episode, Thy Flesh Consumed, and the expansiveness of user-made .WADs.

Knee-Deep in the Dead and The Shores of Hell are simplistic, but brisk, constantly keeping the player flowing through the enticing action that Doom's menagerie of demons and arsenal of weapons provide.

Inferno offers some more complex levels, but at the cost of that aforementioned flow, and can often feel obtuse or bloated.

Ultimate Doom's addendum in Thy Flesh Consumed proves that while Doom's base is strong, what really elevates it is excellent level design. Not every level here is a hit, but most offer compact, yet intricate almost puzzle-like levels that require problem solving above just twitch reaction in combat.

Throughout Doom, there are aspects I question, such as certain power-ups resulting in headache-inducing color overlays on the screen, or the game's obsession with making you wade through puddles of toxic sludge in what begins to feel like every other level, but by playing through the game, it's no mystery it was the sensation it was.