At the Game Developer's Conference, a guy once explained to me how Doom's difficulty settings worked. He was completely utterly wrong. He thought the settings increased the monster's health and weapon damage. In his defence, I was working for Ensemble Studios by then, and I don't think he knew I'd actually built the majority of Doom & Doom 2's levels. Still, it was an impressive display of ignorance. So you know - every item and every monster on a given level had a flag indicating which of the four difficulty levels it appeared on. When you load up Doom and choose Hurt Me Plenty (difficulty 3) then only the objects flagged for Level 3 show up. This includes weapons, ammo, even decorations. Sometimes we'd just add more enemies. Sometimes we'd outright replace them - there might be a chaingunner where a normal zombie used to be. Sometimes there was less health or less ammo. Sometimes we'd add MORE ammo for a harder difficulty. (For those wondering, yes we could flag a given object to exist across multiple difficulty levels.) Since we started with 4 difficulties, this means when I built a level, I built it four times. I had to playtest it four times as much. I had to balance the ammo and health and bonus items four times. But there is more to this story. When we finished and released Doom, then within a couple weeks we saw a guy boasting online how he'd beaten it on Ultra-Violence (our toughest level then). In our innocence, we thought it would take way longer. I mean, we KNEW the levels like the backs of our hands, and Ultra-Violence was a challenge for US. Now, I don't know how long the guy spent playing the game - I'm guessing hours each day. But it got John Carmack mad. Now our problem was that we couldn't go back in and add a new difficulty. I mean, we could, but it was an awkward waste of our time to go in to EVERY ONE of Doom's 27 levels and edit each one yet again to rebalance all the items and monsters. Again. That would take weeks. We needed a quick fix. So John Carmack did it programmatically. He had the monsters move and shoot at double-speed. He almost completely removed their idle animations, so they'd throw a new fireball the second they hurled the first. He also increased the speed of their fireballs. Imp and Cacodemon fireballs move 50% faster, while Baron of Hell move 1/3 faster. This made for a relentless barrage. One bad effect of this means of re-balancing is that since we didn't actually re-playtest the levels they vary WILDLY in how impossible they are. For instance, suddenly the toughest of all Doom levels became E1M2. It's packed with zombies and shotgun zombies who have instant reaction speeds and since you can't dodge bullets, they suck your HP down to 0 before you can find a decent weapons. At least in Doom 2, the hardest level on Nightmare was the final boss (I did this one). The cacodemons knock you off the ledges with their endless fireballs and you can't really get your act together. I don't even know if it's possible without a cheat code (I never tried). Let me know in the comments. Anyhow, that's where Nightmare difficulty came from. But hey, at least I didn't have to re-balance every level again on a fifth difficulty.