Everything began with an unloved game jam prompt. In August 2012, fresh off The Binding of Isaac's enormous success, Edmund McMillen took part in Ludum Dare 24. The theme for that competition's entries was put to the vote until, after five rounds, the organisers were left with a top 20. "Plus one joke theme," according to the now-deprecated webpage: "1,000 kittens."
This theme came dead last in public voting. McMillen took it as his inspiration regardless, and together with Team Meat co-founder Tommy Refenes began prototyping a game that featured not just 1,000 cats but – according to an early marketing tagline – 12 sextillion of them. "I just love the idea of going infinite," McMillen says. This number was achieved using a method McMillen had developed with his previous game, using Flash to add multiple layers of ‘animation clips' to augment the character's look. "I really liked how he'd change visually as you went," he says of Isaac. "And that was the initial spark. I could make a cat-like shape with a bunch of different clips for arms, legs, bodies, heads, whatever – randomise them. We could simulate those as DNA, and then you could have the cats breed and spit out all these different combinations."
It was a simple idea, but one McMillen found himself "pretty obsessed with". Dig up his blog posts from the time, and the enthusiasm is evident. "It's been very hard to keep this one a secret," he wrote in the announcement post for Mew-Genics, as it was then known. This was in October 2012, just weeks after the game jam.
Of course, the game would go on to be a success that dwarfed even Binding of Isaac, selling a million copies in its first week of release. But it'd take an awfully long time to get there, and at the beginning, in the case of everyone but McMillen, the idea was as unloved as the prompt that had spawned it.
From Mew-Genics, to Mewgenics
A year on from that fateful Ludum Dare, McMillen and Refenes had a version of the game they reckoned they were ready to show in public – albeit not one that looked anything like the Mewgenics we know today. "There was a house you could upgrade over time, where you could interact with the cats, and they'd have these wacky animations, and then use those cats as avatars in minigames," McMillen recalls. "I think we only had one functional minigame, the cat fights, which was this Pokémon-esque turn-based combat game."
That was the version of the game Team Meat intended to demo at PAX Seattle in August 2013. At the very last minute, there was a change of mind, and the pair attended without the playable build they'd announced. It proved to be a turning point in more ways than one.
"When we came back from PAX, Tommy was like, ‘I can't work on this anymore," McMillen says. "And I begged, I pleaded… but it was cancelled. That was it. There was nothing I could really do about it." At the time, Refenes' sole interest was making a follow-up to Team Meat's hit platformer, which would be announced the following August as Super Meat Boy Forever. In the announcement blogpost, McMillen explained: "Mewgenics is not cancelled, we just put it on hold to work full time on this one."
Super Meat Boy Forever would eventually be released in 2020. By then, McMillen was no longer part of the project, or Team Meat. "I realised we wanted very different things," he reflects now. At the time, though, he kept trying to make the Mewgenics dream a reality. "I was still pitching it to Tommy before I left. I was trying to wrap it in a genre that I thought he would be excited about."
One version McMillen remembers was closer to Spelunky, the game that had inspired his step into Roguelikes with Isaac. Another came from his longtime friend and collaborator Tyler Glaiel, a fellow traveller in the '00s Flash webgame scene. "I remember suggesting back then: ‘Why not try Final Fantasy Tactics-style combat instead of Pokémon combat?'" Glaiel says. "Because they're on the same tree, kind of, but you can do a lot more with the positioning stuff." McMillen laughs: "Yeah. One has depth, and the other has the illusion of depth."
After McMillen left Team Meat in December 2016, he and Glaiel were free to work together again – not on Mewgenics, but on action platformer The End Is Nigh. "The whole time this was happening, I kept pushing these Mewgenics-like ideas at Tyler," McMillen says. One was a game featuring randomly generated aliens that the player could breed.
"There were some differences," Glaiel says. "The animation was weirder, because it was more rigged and physics-y, because the body parts were so different from each other." Still, Glaiel knew exactly what his old friend was up to. "You just wanted to make Mewgenics, I could tell," he says to McMillen. And, after working on the alien prototype for a week or so, he concluded: "This is a worse theme than cats." After all, it lacked the heartfelt motivations that had driven McMillen to this idea in the first place.
Because, despite the way Mewgenics encourages you to treat your cats, the developer is an animal lover. "Our first cat, Guppy, was a stray kitten that was abandoned in a planter box by its mother," he says. Guppy passed away in 2014 but was immortalised as a powerful character transformation in The Binding of Isaac. "And then, with money and whatever else, my wife was like, ‘I want a hairless cat.'"
That was his introduction to the world of designer cat breeding. The McMillen family have owned three hairless cats, the latest "a Russian import named Puggy", but he has little love for the industry that produces them, which he feels prioritises aesthetic features – unusual ear shapes or fur patterns – over the health of the cats themselves. "They're breeding for rare drops! It's the same as in the game!" These words are delivered with a righteous fury that seems to have burned in McMillen through the long journey from inception to release. "And that's what really kept me going throughout."
Hairless horrors
After The End is Nigh shipped in 2017, McMillen and Glaiel were finally ready to start turning this idea into a reality. The game's themes were already in place, as was a visual style and a cast of characters: Dr Beanies, Butch and the rest made their debut in blogposts dating back to 2012. You can even find an early version of the theme tune, recorded by rock duo Ridiculon. The one remaining question: what kind of game was this going to be?
"There's a bunch of prototypes from that era," Glaiel says. One was "a Castle Crashers-esque beat-'em-up". That proved difficult given the developers wanted multiple characters in play at once, "because you can tell more interesting stories when it's a group, instead of just one cat". So how were they to make that work in real time, without taking the multiplayer route? Glaiel remembers some possible solutions: "What if you have one main cat and a bunch of smaller follower cats that repeat what you do? What if it's just three AI-controlled cats, and one main cat you're controlling?"
They also experimented with making a StarCraft-style RTS. That proved to be "a nightmare to control," Glaiel says. "There was no strategy. You couldn't dodge attacks because you didn't have direct control. You could only tell things what to do, so if an enemy was going to hit you, it had to have a five-second windup so you had time to click your cats and move them away."
"It's funny," McMillen says to his friend. "When you describe them now, they don't sound bad." Glaiel agrees: "Both of those prototypes, I think there's a world where we executed on them and made a decent game. Probably not as interesting as the one we ended up making…"
"Both of those prototypes, I think there's a world where we executed on them and made a decent game..."
Tyler Glaiel
The game they ended up making, of course, came back to Glaiel's old suggestion of turn-based tactics. Years earlier, McMillen had brushed off the idea because it would mean an awful lot of work. "And that's true," says Glaiel, having worked on this incarnation of the game from 2020 to 2026. "Certainly can't disagree with that!"
Part of what made things so complicated was that the game had two layers. The pet sim McMillen had started out with, and the tactical RPG Glaiel had introduced to the mix were dependent on each other to function. The specifics of the cat-breeding system couldn't be nailed down until they'd "made sure the combat was working fine," Glaiel explains. "Because the breeding doesn't really matter if you don't have anything to use the cats you're breeding for."
Their solution to this sticky situation? "We just kind of worked on the systems seemingly at random, until they were mostly complete, and then we connected them all together," McMillen says. "And we never spoke about it, really." This was only possible because of the pair's dynamic and shared sensibilities – and even so, it didn't always work out. "We were just kind of hoping our assumptions were correct," McMillen says. "And some of them were wrong."
One of those dead ends involved the game's cast of weirdo NPCs, who'd been around since 2012. What role they'd actually serve in-game didn't become clear until late in development, McMillen says. "Initially, the idea was that you'd upgrade the NPCs with money." They didn't like the player grind that encouraged, and so he searched for an alternative before eventually realising: "Wait, we have a resource right here. We have cats."
In the final game, any kittens with undesirable traits or elderly fighters past their prime can be dumped on these characters in exchange for rewards – something else that wasn't obvious at first. "Dr Beanies giving you side quests, we figured that out less than a year before the game came out," Glaiel says. "Before that, he'd give you an ability needle, with a random ability from one of the cats you'd sent him. And they'd just clog up your inventory, because most of the time it would just be some colourless ability."
Rather than ditching this idea entirely, however, the developers found another home for it – you can obtain ability needles by beating an optional hard route with one of the late-game areas. "There's a lot of that," McMillen says, "where we'll do a system, it'll be really in-depth and interesting, but maybe doesn't work how we intended. We don't forget it's there. We make a mental note, and in the future, when there's a hole we need to fill, we can just pull that shit back out."
This resulted in one of Mewgenics' greatest strengths: its wealth of ideas, where something originally designed as a headline feature might be tucked away where relatively few players will ever find it. "The people who have been playing it hardcore since release are starting to go, ‘I've got 300 hours in this game, and I've never seen this before'," McMillen says. "There's a tremendous amount of this very rare, difficult-to-find shit that we just kept adding. And those details, even though they're somewhat needless at times, create the illusion this is a living world. That's where the magic is with games like this."
That's something McMillen learned with Isaac, over the many post-launch updates developed alongside Mewgenics' very long journey to release. He admits to feeling competitive with his own game: "The bar got raised so high, especially with where Isaac ended. Going into this [project], we had this not-so-indirect comparison of: ‘This is what we're going up against – this game that has ten years of development content in it already'."
Although it's less than a year old, Mewgenics already feels like a game that's undergone a similar decade-long period of iteration and expansion. And with many millions of cats having already been fed through its meat grinder, McMillen and Glaiel have been able to see that old magic at work over and over again.
"There are things we forgot were in the game," Glaiel says. He describes an online discussion where a player was trying to figure out why a particular unit had been blinded in battle. "And it's like, ‘Oh, he was standing on a dirt tile, and he used a wind ability – and that apparently blinds them'. I don't remember putting that interaction in the game."
This is apparently news to McMillen, too: "I remember we briefly joked about it very early on, when we made those tiles. I didn't know it was ever implemented." The pair begin trading possible interactions, wondering if they're in the game or not, or whether they should be. Do holy-element powers cause grass to grow, the same way water does? Gravity powers crush grass, and immobilise flying units – but what about water? And how does it interact with rocks?
"I don't think it does a lot," Glaiel replies. "It's mostly just there as a tag for Psychics to interact with." And that gets them onto the ways the elements' effects could be expanded in future. Glaiel, on the spot, starts musing: "One of our DLC classes could probably do stuff with gravity…" When it comes to "going infinite", McMillen has clearly met his match.
This Mewgenics article was first published in issue 424 of Edge Magazine. To get more like this delivered straight to your doorstep (or digital device) consider subscribing to Edge.
