Every gaming blog is required by Laws of the Internet to attempt answering this question, so here I am fulfilling my duty.
The Wrongest Answer
The totally wrongest answer is called "Berlin Interpretation", and it's just a completely unfocused list of features early Roguelikes shared. Half of that list is just limitations of computers from early days, nothing to do with game design. It's the worst. Let's just get it out of the way.The Wrong Answer
So my first idea was that Roguelike is Procedural Generation plus Permadeath. This definition is a trap. Minecraft on Hardcore mode is not a Roguelike.Procedural Generation
Procedural Generation is a spectrum, and there are very few games with fully fixed content. Even Tetris gives you random tiles. And no game is completely random, whatever that might mean - there's generally some fixed outline, which is filled by some combination of fixed content (often initial or final parts), random selection of handmade elements, actual procedurally generated content, and purely random events during the game.For games where "map" is a meaningful concept, a reasonable dividing line is having procedurally generated map.
So Skyrim (completely fixed map, mostly fixed quests, some random encounters, random quest variants) is definitely out. Minecraft (random map) is definitely in, at least by this criterion.
So what about a game like XCOM: Enemy Within? It's a series of battles, each on a randomly selected handmade map, and with procedurally chosen aliens. If we see the whole XCOM campaign as a map, and each battle as like a dungeon level, it feels like it's mostly in. XCOM 2 went with even more procedural generation, and so it feels even more Roguelike than XCOM 1.
This criterion gets a bit fuzzy, with most games being in between, but it's a fine part of any definition of a Roguelike.
Permadeath
So what about Permadeath? It has absolutely nothing to do with being a Roguelike or not, other than very indirectly.Minecraft definitely has procedural generation. It has Hardcore mode with permadeath. Is it Roguelike? What if Civ5 had ironman mode like Paradox games? Nowadays all kinds of games have optional permadeath mode, since it appeals to a subset of players, and takes very little effort to add.
It's pointless to argue that permadeath only counts if it's not optional, or that these games were not "designed" to be played in permadeath mode, like author's intent can be meaningfully known. Saying Minecraft on Hardcore or Civ5 on Ironman counts as a Roguelike, while Minecraft or Civ5 on Normal don't just shows how badly this attempt at a definition fail.
Permadeath is a dead end.
Tactics Based
Before I get to what to consider instead of permadeath, we need one extra criterion. Roguelike must be primarily tactics based, not dexterity based.There's no amount of procedural generation that's going to make a game like Modern Warfare, or Super Mario a "Roguelike".
So games like Rogue Legacy are not even remotely Roguelike.
I'm not being arbitrary here. Tactics-based games usually have partial or full procedural generation, and dexterity based games often have extremely fixed content. Different genres have different requirements.
I'm using very broad phrasing here. Turn based systems totally work, but "Real Time with Pause" systems like Total War or FTL are just as fine.
This is of course also a spectrum, as games like Factorio are overwhelmingly tactics based, but they still have minor dexterity based combat system. Which by the way should be removed from the game completely, and it can when starting a new game.
Short Playthrough Time
So now that we killed permadeath as a possible criterion, let's see what we can use instead.Why would Civ 5 on Ironman mode not be considered a Roguelike? Mostly because it's intended to be played in long campaigns.
The key characteristic of a Roguelike is that it's meant to be replayed over and over. Short playthrough time is key to this. You can't just "start over" a 20h XCOM campaign or a 100h EU4 campaign.
Because playthrough time is short, permadeath by default is reasonable. I find Paradox idea of forcing ironman mode on games with very long campaigns like EU4 and CK2 totally idiotic. Bad RNG, misclick, or game bug destroying a run that takes 30 minutes anyway is totally reasonable. Bad RNG, misclick, or game bug destroying a campaign that takes 100h to completely is just shitty game design - especially since longer campaigns give far more opportunity to game breaking bugs to happen.
And yes, if you took a Roguelike game, lowered difficulty (so permadeath can happen but it wouldn't be a common occurrence), and extended its dungeons to 10x current size, at some point it would stop being a Roguelike.
On the other hand, if you took a not-really-Roguelike game, and just made a short mode out of it, it'd probably become Roguelike. Like let's say if XCOM had a mode where you do just 5 random battles on procedurally generated maps, it would be basically a Roguelike.
And if you took a Roguelike, and added a save game system, it would still be a Roguelike.
Into the Breach is another interesting case. It's as Roguelike as Roguelikes get, but it doesn't have total permadeath. You get one time reversal per battle (two per battle with one of unlocked pilots). You're playing time travelers, so it makes perfect sense, it's a highly controlled feature, and removing it would just make game more frustrating pointlessly.
Links Between Runs
This also answers the mystery why pretty much every Roguelike nowadays have some ways to keep a bit of stuff from previous runs in your new runs.This definitely goes against the idea of permadeath, but it makes so much sense when you realize that short playthrough time is the key. Letting you keep an unlock or a small bonus from previous run simply incentivizes starting over.
Examples
Of games mentioned:- DoomRL - Roguelike
- FTL - Roguelike
- Into the Breach - Roguelike
- XCOM - not a Roguelike (but could maybe have a Roguelike mode)
- Civ5 - not a Roguelike
- Diablo - not a Roguelike
- Factorio - not a Roguelike
- Rogue Legacy - not a Roguelike
- Skyrim - not a Roguelike
Full Definition
Roguelike is a tactics-based game with mostly procedurally generated content, meant to be replayed over and over thanks to its short playthrough time.It frequently features permadeath, and there's often some links between runs, but these aren't essential features.
Why any of that matters?
You might be puzzled why so much has been written on this subject. Game design is still in very early stages, and even when we make a game that works, it's mostly by accident, and even best games are still full of design fails.Having more clarity on which features go or don't go together and why can only improve this sorry situation.
And of course there will be games that break any established rules, but it's really helpful to understand why those rules exist in the first place.
Having no clue and just blindly borrowing features from mismatching genres just got us to design failures like ironman mode in EU4, and dexterity based combat in Factorio - otherwise great games.