I first discovered They are Billions a few months back, but even then I was pleasantly surprised by a few things: the level of polish, especially for a game still in development; the easy-to-understand yet complex-to-execute rules that directed the building of your colony; and the difficulty. There may be billions, but all it takes is one misplaced zombie to turn most of a colony into a screaming horde of flesh-eating maniacs.

I got hooked quickly and lost countless hours trying to come up with the best strategy to defend my base. I lost many rangers, soldiers, colonies, and a whole lot of sleep trying to fend off the hordes of ravenous undead. It was a rollercoaster of emotions, from excitement at the sight of the final wave to frustration when my three-hour-old colony got crushed, and triumph as I finally beat the first map.

It's easy to become frustrated and feel like They Are Billions' survival mode is impossible to beat unless you're a StarCraft veteran or some insane mutant keyboard ninja, but I’m here to give you a few tips and points on how yours truly managed to defeat it.

Use the pause button to think 

Don't worry about destroying your spacebar as you frantically pause and plan your next move. Use this time to check the calendar to know how much time you have left before the next wave. Never leave a portion of your base unattended, use the patrol move (the shortcut is P) on your units so they cover the most area, and to be sure no zombie can pass through your defenses. 

Slower zombies roam the interior of the map, so they're easier to catch with your speedy Rangers. But don’t forget that special infected roam the northern and southern side of the map. If possible, expand to the east or west first to avoid activating them too early.

Expand to focus on generating gold 

You're going to need billions to defeat Billions. Gold is used for everything: units; research; buildings; defenses. You'll need not just a big bank (through Warehouses, which store gold and resources), but a strong income rate to allow you to recover quickly. 

Treat land as gold. With more land you will have more food and therefore be able to build more houses. You need to expand early. If your Rangers spot some free stone on the map, save it for a Soldier Center—more units means more area you can own and protect. As far as research goes, if you don’t have good food revenue you should aim for Farms first (if your first mayor offers you Farm tech, elect them immediately), otherwise aim for Cottages first.  

Urban planning 

The very first thing you want to build is either a Fisherman's Cottage or a Hunter's Lodge, depending on your starting position. You'll need the food. Follow that by dropping seven Tents to get your worker numbers up, and also so you hit the first mayoral election as soon as possible. Make sure to leave room on the green grass for Farms.

The Market and Bank buildings should literally be at the center of your housing so plan for that from the start. The Bank increases the gold output of surrounding buildings while the Market reduces the food cost of anything in its radius, and you should build a massive, dense housing district around them. You can only have three such districts so plan carefully on where you will put them for maximum efficiency. Markets also let you sell excess resources and though they'll do so automatically, it's more efficient to do so yourself.

Don’t build advanced buildings if you can expand and find resources elsewhere. Most of the time their steep upkeep cost will only hurt your economy.

Rangers early, Thanatos late 

Tiptoe in the early game but be ready to rumble in the late game. Zombies look so inoffensive with their low pool of HP and slow shuffling, right? Wrong! They are lethal death machines that will swarm you. Some of them can run, some of them can hop over walls, some even spit splash-damaging acid. In the early game you don’t want them banging at your door and so you want to make as little noise as possible. Thankfully, humanity have given you the perfect tool for this job: the Ranger. Armed with a bow and arrow they deal with the infected threat as silently as possible. The Sniper is also a viable option, but they generate much more noise, and will eventually attract lots of undead if they're firing often. Be especially careful around a Village of Doom—the ghastly haunted houses that contain a large but finite amount of undead—as an early army will not prevail.

Once you've built up a reliable defense, put together a mobile fighting force for clearing the map. I like a mixture of Rangers, Snipers, and Thanatos, and a Titan or two if you're economically booming. You can hit the C key to command these units to 'chase,' or seek and destroy any and all zombies on the map, though you'll still want to micromanage your forces, as they won't do much to arrange themselves intelligently. It's important to clear as much of the map as you safely can before the final wave, where any spare undead will join the finale. Map hygiene might make all the difference.

Layer your defenses 

Never neglect your defenses. You think that this massive wall protected by a plethora of Executors and Ballistas will be enough? Think again. Assume the worst, and focus on reinforcing your weakest points first. Always layer your base and use chokepoints to organize those layers. By the final wave, zombies coming from any direction should encounter at the very least three layers of stone walls protected by Executors. 

Executors are your best pals. Build them everywhere. Don’t get tempted by the Shocking Tower, which costs tons of energy and won't give you the DPS you need in the endgame. Consider spike traps as a luxury, too: they're resource-intensive and not durable enough to be of much use. Ballistas are all good in the early game and have the advantage of being upgradable to Executors. But in the end, Executors are your BFFs, paired with layers of stone wall. Build them everywhere, pause the game when there’s a breach, and delay the horde with layers after layers of walls. Only then will you prevail.

The steampunk zombie survival RTS They are Billions has become a sensation over the new year break. You build bases, suck resources out of the terrain nearby, and then produce as many soldiers as you can to withstand the increasingly fervent attentions of the zombie hordes nearby. Instead of hunting down an enemy base, all you have to do is survive to win. Sounds easy, doesn't it? It's not. It's horribly, horribly hard.

As the game kicked my ass repeatedly earlier today, I was reminded of a few different games that have toyed with this formula before. It looks like an RTS, thanks to the base building and unit production, but plays like a tower defence game. Here are a few other good games that explore this curious in-between space, mainly by throwing thousands of bad guys at you.

StarCraft 2: Wings of Liberty

If you want to hold off hordes of enemies as they run riot through your base, then the Terran vs. Zerg singleplayer missions in StarCraft 2: Wings of Liberty are a good choice. In Haven's Fall the Zerg move to infest human settlements as your Terran army tries to stop them. In Outbreak you start surrounded by zombified settlements that vomit out hordes of aliens every time night falls—sound familiar? StarCraft 2 marines look pretty similar to your basic gun troops in They are Billions, and TaB's watchtowers are basically StarCraft 2 bunkers.

Desktop Tower Defense

There is a strong tower defence element to They are Billions. Waves of creeps attack from different directions and mindlessly throw themselves against your defences until you or they are toast. Most tower defence games, like Defense Grid: The Awakening, funnel creeps into a set path that you cover with gun turrets. Desktop Tower Defense and They are Billions share a more open format. In both games you funnel the creeps into kill zones using the most efficient unit/tower placement you can invent. DTD has an advantage because it's free and you can play it in your browser.

Revenge of the Titans

Revenge of the Titans features giant city-wrecking Pac Man ghosts instead of zombies but, deep down, they hate the living all the same. This wave defence game has a much more developed resource system than Desktop Tower Defense, and you can place buildings with a bit more freedom than games like Defense Grid. Plus it has satisfying lasers that you can control with your cursor.

Infested Planet

Another RTS about fending off thousands of enemies at once, Infested Planet sets your desperate last stand on a luminous neon world swarming with hundreds of alien creatures. You command a squad of five marines on a desperate mission to find a chokepoint and throw down some turrets. If you hold off the horde long enough you can start to spend resources and upgrade your squad's weapons. Look out for the hives nearby—they spawn increasingly tricky monsters as you fight through the campaign.

Project Zomboid

Project Zomboid is a survival game rather than a RTS game, but you're still gathering resources, building defences and fending off massive hordes of zombies. Death is inevitable, but it's the horrible, lingering journey full of disease and suffering that counts. Project Zomboid has been receiving new features since its Early Access debut in 2013, and has evolved into a deep, clever sandbox game that, like They are Billions, will kill you ruthlessly over and over.

Death comes as a sudden, unpleasant surprise in They Are Billions. It's a real-time strategy game strongly reminiscent of late-'90s classics like Age of Empires or StarCraft, but rather than a symmetrical battle against other civilizations, you struggle against a horde of mindless zombies. The slightest breach in your defenses can mean that your base is infected, causing an explosive—and usually game-ending—surge as the zombies convert your helpless citizen residents into more zombies. A single shuffling zombie that sneaks past a poorly placed wall or into a new expansion can lead to a snowball that rolls over your entire base before you know it.

Director Jesús Arribas took the time to speak with me about what led Numantian Games to make a game about a zombie plague, as well as how their own viral growth ended up getting out of hand.

Questions and answers have been edited for brevity and clarity.

PC Gamer: They Are Billions is quite a departure from Numantian's previous game, Lords of Xulima. That game was a very traditional role-playing game in the vein of Might & Magic and Wizardry. How did Numantian Games go from straight-laced fantasy to this offbeat survival strategy game?

Jesús Arribas: They are very different themes and genre, but if you play both, you can see the same author’s touch, like the same painter who draws two different scenes.

Real-time strategy games have followed a similar trajectory as RPGs. The classic titles were very challenging and hard to master. You had the freedom to play the missions the way you liked. You were free to lose if you made stupid decisions. But at the same time, when you won, it was very rewarding. We have always been fans of RPGs and strategy games. We tried bringing back that feel of the old-school RPGs to the present with Lords of Xulima.

We have the same goal with the RTS genre and They Are Billions. You have to manage the economy and the resources, expand the colony, build an army, attack your enemies to clear areas, reach new resource sources, and of course, defend your base from the infected hordes. And this is just the Survival Mode. We are working on the Campaign which is a bit more similar to classic RTS, with a big story, tactical missions, and much more content.

Losing in They Are Billions happens very fast. Any failure means zombies convert your built-up resources into more zombies very quickly—and you even warn players about this threat. Why does They Are Billions have such a sharp death spiral into failure and game over?

JA: In this game, the zombies are not just an excuse to have an enemy to fight—they are the central pillar of They Are Billions. This is not about killing enemies. This is about combating the infection—the same infection that destroyed the world and is the main threat to the new survivors. You could have a massive colony and suddenly find it in ruins because of a single infected breaking through. Fear and danger are constant.

The infection is the real threat to the survival of the colony. We know many players can get frustrated because of this. There is no other game like this. So, when a player loses their first colony because of a single decrepit zombie, they can hardly believe it. It is part of the fun. Nevertheless, a full game can last 2-3 hours if you survive all the way to the end. It is a game about surviving and improving your survival skills, not winning. If your score gets higher every time you play, then you are doing great.

Most developers tend to think that they must take care of the players, that they need everything easy and straightforward. But we don't. Players are much smarter than we give them credit for. When they are challenged with a game like this, where there is no hand-holding at all and even losing is fun, they actually love the experience.

Shooting zombies can fit into all sorts of settings, from traditional high fantasy to far-future science fiction. Why steampunk? What about Victorian aesthetics made them feel like a good fit for a survival game?

JA: From a development point of view, we thought that it would be super interesting to mix a steampunk society with their low technology with a zombie apocalypse. Watching the Titan, a steam machine driven by an educated gentleman with Victorian manners, destroying zombies is something worth experiencing.

Because it is set in the future, the 22nd century, one may question, "What has happened to the civilization and the modern world? Why has technology gone backward to a 19th century level? Where did the zombies come from?" You have to play the campaign to find the answers. 

A significant departure from Numantian Games' previous work is the shift from crowdfunding on Kickstarter to launching straight to Steam Early Access. Why didn't you head to Kickstarter a second time? 

JA: Instead of launching a Kickstarter, which requires months of work and preparation, we decided to do something softer and just to launch a campaign directly from our official Numantian Games website. We thought that it would be a great idea to let the backers play the Survival mode to collect feedback and test the game, [so] we then added Steam beta keys to the rewards and released the [crowdfunding] campaign on Halloween. 

And what happened? People started to purchase the game, play it, and upload videos to YouTube. The game went viral very fast. So fast in fact, that we ran out of Steam beta keys in one week for the lower tier, then in a few days the next tier, and so on... 

In two weeks, we ran out of beta keys—we originally had 10,000. We asked Valve for more beta keys, but we were unable to acquire any more keys until we released the game on Steam. We ended up in a situation where big YouTubers were playing the game, but people were unable to purchase and play it as well.

We made the decision to release the game with just the Survival Mode on Steam Early Access. Then we could work on the main game mode, Campaign mode, during the Early Access phase. We had to release the game about six months earlier than what we would have liked. Fortunately, the launch on Early Access has been very successful. We are still on the top seller list since the launch, the comments and reviews are very positive.

Where do you plan to go from here? What's your first priority to add (or fix) in the near future? What are your goals before leaving Early Access? And what are your plans once They Are Billions reaches 1.0?

JA: We are working in parallel on two different fronts. First, solving bugs and fixing problems and hardware compatibility issues. And second, making the game multi-language. Our goal is to have the game in eight languages in just a few weeks. The game is selling well all over the world, so we want as many players as possible to enjoy it in their own language.

After that, we are working on more features for the Survival mode, new buildings and zombies. [Another upcoming new mode is] Challenge of the Week. In that challenge, all the players can compete for getting the highest score playing the same survival map. We think it will be very funny as even the developers will participate in the challenge every week.

Finally, we continue working on the campaign. Thanks to the success of the game we can invest more in creating a memorable and epic campaign. Stay tuned!

They Are Billions is currently available on Steam in Early Access.