How Do Infested Planet’s Mutations Work?

This is The Mechanic, where Alex Wiltshire invites a developer to help him put their game up on blocks and take a wrench to hack out its best feature, just to see how it works. It’s about the sweat, grease and genius behind the little things that make games special.

Infested Planet [official site] is an RTS that channels Starship Troopers. You control a small squad of soldiers fighting their way through caverns of hives that endlessly churn out insectoid bugs. The body count reaches tens of thousands as you capture the hives, steadily gaining and conceding ground to make best use of your limited resources. It’s an intensely dynamic game of observing and controlling flows of bugs and continually respec-ing your forces, because you’re always up against:


Singleplayer strategy games are struck through with a weird tension. You start out getting to know their big boxes of tools, understanding their strengths and weaknesses through experimentation, trying out this against that and searching for optimal solutions. This is the sweet spot, where you’re partly anxious because it feels like you’re always on the back foot, but you’re also enthralled because you’re coming up with new plays and being rewarded with success and chided by failure.

But when you find those solutions, experimentation withers as you start to use them over and over. Falling back to what you already know means you’re probably not making interesting decisions any more, and at a deep level the game kinda stops being fun. You’ve stopped learning, and levels become rote exercises in performing a routine to get to the goal.

Designers know this, of course, and the typical response is gimmickry. In games like StarCraft II it’s cool gimmickry, I’ll stress, where levels are often one-off scenarios that equip you with specific tools, handicaps and enemies which force you to explore new strategies. That makes the singleplayer levels in SCII super fun, and also, once played, not really worth returning to.

Infested Planet goes about it differently. Instead of pre-determined situations, it has a feature called mutation, which dynamically changes the composition and abilities of the enemy in order to counter and disrupt yours. It feels fresh and challenging, and it’s also part of a general movement in games where they’re becoming more responsive to player strategy, such as the way delivering headshots in Metal Gear Solid V eventually prompts the enemy to start wearing helmets, and that enemies will start shooting down Gargoyles in the Arkham Batman games if you use them a lot.

In Infested Planet, mutations, which generally trigger when you destroy an enemy hive, might introduce new unit types or effects. So, you might have equipped a couple of your soldiers with flamers, which are short-range, wide-angled weapons that are massively effective against standard bugs, knocking them back and dealing high damage. But then the bugs might get the mutation Bombardment, which increases the range of their towers, or Impact, which adds knockback to hits from towers, or Neurotoxins, which make tower hits slow your soldiers. Your flamers suddenly can’t close range on hives, and you’ll need to retool and rethink, fast.

Infested Planet’s mutation amounts to a lightspeed and in-game version of StarCraft’s metagame, in which you are always having to react and reconstruct your approach in accordance with what’s being flung at you. You can’t settle on a single strategy, not even during one level, because sooner or later the game is going to deftly undercut you. Designer Alex Vostrov puts it like this: “I know you love minigunners, but try something else. There’s another solution here.”

The entire game is founded on this precept. Vostrov was subconsciously inspired by Red Alert 2 and the patterns of singleplayer RTS design that had been laid down by Dune 2. “What does the opponent do? Sends little trickles of attack squadrons, right? It’s very pronounced in Westwood Games, where it wouldn’t really play against you. Scenarios were set up where it had gigantic bases; if it was a player it’d be able to crush you. So it’d send out trickles of units, and that’s what bugged me, and what started me off.”

Because mutation faces you with shifting challenges, you need to be able to shift your forces, too, something Infested Planet addresses through a novel approach to resources. “A lot of Infested Planet can be just read as things that frustrated me about modern RTSes,” says Vostrov, this time bugged by the way you can generally win them. Because the AI doesn’t rush you, a fine strategy is to mine resources out, build a massive army, and sweep the map. “Any challenge can be met by waiting another 10 minutes. Which kind of sucks.”

In contrast, Infested Planet awards resources in proportion to your captured hives, but once gained they sit in a reusable pool. You can buy a sniper and then change it into shotgunner, or build a turret and later sell it for its full value. A good player (someone who is not me) constantly shifts roles – going scout to move quickly across the map, before switching to shotguns and snipers on arrival.

You’d expect that mutations are controlled by a system that watches your performance, and you’d be right, but only on the highest random skirmish difficulty (Terminus) and in the last mission. That’s because it’s really hard to play against.

For the rest of the time, enemy mutations are actually just random. But it doesn’t feel like that at all. Every mutation feels like a direct result of your tactics. Vostrov wasn’t expecting this – he merely wanted to know mutation was actually fun before he invested the time in creating an AI system to manage it (“I’m a lazy designer and programming these things is hard”). But it worked. “From a designer’s point of view, what’s the point of complicating things, unless you’re doing it to serve the player?”

But he wanted a special final level, so right at the end of the game’s development, he created that system. Vostrov shows me a debug overlay listing numerical weightings to certain stats and spawns some miniguns. The ST_MU_MINIGUN value starts to steadily tick upwards, and as it does so, values for certain mutations start to rise, too. Soon, Clone Strike reaches the top. It’s an effective response to powerful minigunners because it spawns clones of your own squad against you, gaining your power. The game knows this is effective, because it’s loaded with tables comparing weighting patterns to effective counterstrategies.

The steady nature of the weighting system means it’s measuring your performance over periods, so if you’re nimble enough, you could feint by sticking with one set of weapons, then quickly attack with another before it can mutate its counter. Still, it’d be plenty strong enough to raise some horrible mutation combinations against you if it wasn’t held back by mutations having point values so, as Vostrov says, “it can’t pig out on the really powerful ones.”

It’s still too hard for inexperienced players, though. Vostrov made Infested Planet more or less alone and he did most playtesting himself. “Which was probably a mistake,” he says. “The problem with designing the core system yourself is that you know the game perfectly, so you don’t have the confusion factor.” He’d add a mutation or weapon and constantly ask questions: When would he use it? When would it be the worst choice? Is it too good? Are there not enough things countering it? “They would gnaw at me internally: ‘Aaagh, it’s too good!’”

The results don’t show that pressure. In fact, the asymmetric nature of the game – your few against thousands – covers most rough edges. It’s a hard game, but more than worth persevering with. After all, it’s rare to play a game that responds so intimately to your actions.


  1. OscarWilde1854 says:

    “This is the first entry in a new column called…” – This isn’t the first.. this is the same as the Alien lockers! Both fantastic articles! I quite enjoy seeing an in depth look at the thought processes behind certain mechanics and how they evolve

  2. Arathain says:

    Infested Planet is a very good game if you’re a single player RTS fan tired of the rut formed by C&C and Starcraft. It manages a neat trick of being filled with constant action, while allowing the player to stabilise a defense and plan a fresh approach. It rewards aggression and expansion while creating situations where retreating and ceding ground is sensible, and comebacks are always possible.

    Allowing the player to remix their upgrade points at will without penalty is a brilliant decision.

  3. horsemedic says:

    This was great, and a great concept for a feature. Looking forward to more behind-the-scenes explainers of great games (or perhaps a shitty game now and then).

  4. mpb says:

    “mutations, which generally trigger when you destroy an enemy hive …”

    Actually, mutations generally trigger when you capture each additional point, not when you destroy a hive. On some levels, this distinction can be very tactically significant.

    I picked up Infested Planet over Christmas, and have been loving it. Infested Planet makes me glad I decided to start playing PC games again.

  5. subedii says:

    One of my favourite RTS’s. It’s one of the only ones I’ve ever played where I feel like there’s a constantly shifting back-and-forth as things change and adapt and you’re constantly having to rework your strategy on-the-fly to either push out or keep from being swamped.

    In most RTS’s, once you reach the mid-point of the level, it feels like everything else just becomes a steamroll as the enemy becomes weaker and weaker and can’t do anything against you. Here it feels like the enemy becomes their most dangerous the more of their territory you take. You have more resources, but with each capture the enemy becomes more aggressive and you have more area you need to defend.

    Locations can swap hands several times as you jostle for control of the map. You’re constantly re-assigning your fixed resources, swapping between trying to blunt the bugs advance, push out or flank, and take and hold. Your troops will swap in-and-out of different classes with each goal.

    At the same time, the game’s pacing is really good. It’s not a constant, manic rush. There are frequent periods of ‘calm’, where you’ve got control of the situation and are just barely using your scant resources to stem the tide. It’s at those points you need to think carefully and enact a plan to push forward. And you better make the right decisions, because taking that base without thought or the right configuration can quickly see what was a stalemate spiral into a desperate defensive action as you try to respond to a new mutation or an unanticipated flank.

    I really do hope there’s a sequel to this game. Honestly if it was just remade with better graphics and nothing else, I’d still probably buy it all over again.

  6. racccoon says:

    Thing I like about this is its just like the invasion from ants n critters in summer, its never ending & never seems to stop.

  7. Phasma Felis says:

    I really like how it starts with the “a few stalwart space marines vs. 10,000 murderous bugs” premise, only that’s not 10,000 bugs over the course of the entire campaign, that’s 10,000 bugs on the map right now and all of them coming for you like an unstoppable slithering tide. Fighting them is like trying to push back an avalanche with a fire hose. It’s lovely.

  8. coppernaut says:

    Hell yeah RPS. I played the hell out of this almost 6 or so months ago. One of the most addicting RTS experiences I’ve had. Slick, clean graphics and UI. It’s too bad it didn’t get more recognition on the internet.

  9. shevtsov200 says:

    Oh, not again.

  10. HuvaaKoodia says:

    Infested planet is in my top 3 pausable real-time tactics games (Atom zombie smasher and FTL are the other two).

    I played it a while ago when it was cheap on GOG. I didn’t find the mutations that interesting as it was obvious to me that they would start mutating one way or another.

    I can only remember one level which caused me any trouble and that had a scripted overwhelming alien rush in the beginning. The more open levels were much more enjoyable and fair.

    The thing that I’d highlight is the user interface; It is great. Any UI designer or programmer should play IP for that alone.

    • Phasma Felis says:

      This is very true. I was delighted by how anti-micromanagement it is. “I want the entire squad to advance towards this point, stopping as necessary when enemies approach and continuing once all enemies are down” is two clicks; they’ll go do their thing while you build stuff elsewhere. So refreshing.

      • subedii says:

        Isn’t that literally ‘A-move’ all the way from Starcraft 1? I think it may have even been in C&C before it.

        • Phasma Felis says:

          If so, the reason I didn’t remember it is that it was usually an enormously bad idea. IIRC, you wound up with your units spread across half the map, defocusing fire, ignoring adjacent enemies they were strong against in favor of far-off enemies they couldn’t hurt, etc.

          In Infested Planet, you can attack-move a squad and expect them to actually behave rationally and take care of themselves for a while. They mostly stay together; they target grenades effectively; snipers target large, tough enemies rather than overkilling individual swarmers, while shotgunners do the opposite; and so forth.

  11. Napalm Sushi says:

    I’d love to see more games that jarringly change their own systems in response to player action. Feedback like this is what makes interaction feel meaningful and creates the feeling that you’re having a conversation with the game rather than just consuming it.

  12. Bonez0r says:

    Just a note to hopefully prevent new players from getting disappointed by the nature of the game: don’t judge the game based on the campaign alone. The campaign is basically a long tutorial. Every campaign mission you learn some new mechanic or get a new type of marine or weapon. For this reason, the story missions in the campaign are largely scripted (for example, you will not get random mutations here). There are random missions later in the campaign, which are clearly marked as such, but you need to clear part of the story to get to them.

    To fully appreciate the game you need to play skirmish missions. They are highly customizable (or you can choose one of the presets); number of hives, number of defensive towers per hive, number of build points you start with, mutations… you can adjust all that and much more to your liking. If you want to test yourself against other players there are the weekly challenges: three missions ranging from easy to hard with a leaderboard attached.

    You can play a quick and easy map if you only have 15 minutes, or play a larger, more difficult map if you want a challenge and have an hour or more to spare. It’s been my go-to game for a few years now and i doubt i’ll ever get tired of it.

    • Bonez0r says:

      Argh, i forgot to include something. Infested planet deserves much more praise than it gets, it’s a highly overlooked game. The reason, sadly, is exactly what i mentioned above, the fact that the campaign is a tutorial. People who do first impressions of games play the first few missions and don’t experience the brilliance of Infested planet that you only get to see later on. Articles like this one help of course, i was happy to see it :)