Inside the walls, life seems almost normal as workers shuttle resources from mills and farms to their depots, walking down streets lined by rows of cottages, lit merrily from within. Then comes the warning: a horde of infected coming in from the east. Squads of soldiers race from their positions elsewhere in the city to man the battlements. Patrols rush back inside the gates and snipers hunch over their rifle scopes, scanning the eastern approaches.

In minutes, an avalanche of gray zombies will come pouring out of the murk, faster than all the archers and gunners can bring them down. They’ll hit the city walls like a million simultaneous hammer blows. The walls will hold at first, but then it’s race between the scrabbling, relentless claws of the horde chipping away at stone and steel, and the ceaseless gunfire the town’s defenders. If the infected open a breach and reach those happy homes, the entire city will die in minutes, and it won’t matter how many defenders remain alive on the walls to see the lights go out.

They Are Billions is like developer Numantian Games made all my favorite levels from decades’ worth of real-time strategy games into a single game. There’s elements here that go all the way back to Age of Empires 2 and StarCraft, combined with a roguelike-influenced survival game. It’s a game of endless last stands, a fast-paced steampunk World War Z. It’s kind of amazing that it took so long to see a game like They Are Billions come along and make an RTS that distills years’ worth of fan-favorite missions down to their essential elements.

The RTS structure is also what gives Billions its more satisfying and captivating tension over a typical tower defense game. A tower defense level is an endless crescendo—though there might be particularly dangerous waves sprinkled into the mix—and your job is to keep a step ahead of that escalation. Billions, on the other hand, has long movements that alternate between calm, dread, terror, and relief.

Each game of They Are Billions opens on your town center and a handful of troops, marooned somewhere in the middle of a zombie-filled map. To start with, your problems are classic RTS: You scout for the key resources (lumber, stone, iron, and eventually oil) while making sure you have enough food to support an expanding population and enough power for your buildings.

The catch is that each building is a potential time-bomb: Each infected building spawns a small wave of fast zombies rather than the slow shamblers that make up most of the map’s undead population. So your little city can quickly fall apart in a “cascade of failure” as one building’s infection spreads to several more.

So you build walls and towers around those buildings to fortify them against the undead and that will solve the problem of the stray zombies that wander into your perimeter. Any stragglers who get close will claw at the walls until a roving patrol comes to dispatch them. But these lulls and their attendant security are deceptive, because it’s not really stray zombies you have defend yourself from. It’s the hordes that appear on the edges of the map and come hurtling toward your fortifications like an asteroid, with especially monstrous infected in their midst and increasing mass as the herds of stray zombies get pulled along on their wake.

Everything about them is _semi-_predictable, from their timing to their strength to their angle of approach. But not enough that you can make specific plans. They arrive within certain intervals, but not on an exact schedule. They will have certain levels of strength and mixes of monsters, but you can still be caught flat-footed by a wave that contains a ton of the giant “Chubby” type zombies that can soak tons of damage while they batter at your walls (though it’d be nice, for a change of pace, if there were a zombie game that didn’t imply that there is an inherent monstrousness to fat people that renders them into terrifying monsters come the apocalypse).

Once you get word of an incoming horde, you have minutes to prepare your defenses. But most of your work better have been done in advance, because surviving the later hordes requires both carefully architected defense-in-depth and swift action that concentrates maximum firepower at the point of impact.

It also requires, and this is where I struggle, a restless and deeply pessimistic imagination. Which is funny, because I think of myself as a pretty realistic-to-pessimistic person in general, who has no problem imagining every possible way in which things can and will go wrong in my life. (I once paid a very nice, compassionate professional for an hour each week to help me do less of this, in fact.) Nevertheless, They Are Billions keeps bringing me face-to-face with my tendency toward denial and wishful thinking.

After you defeat a horde, the game seems to settle down for a bit. You can get back to expanding your city, building new walls, clearing out sections of the map, and increasing the size of your army. And during those phases, it’s way too easy to think that you’re ahead of the game now. Even if you know from brutal firsthand experience that the next horde is going to be devastating… well, it’s not here now, is it? And in the meantime, look how much more territory you’ve cordoned off behind new defenses! You’ve got even more towers and soldiers now!

That's what always gets me. The minutes slide by without any attacks and suddenly I’m starting to fill-in the territories behind my outer walls, the places I said I wouldn’t put buildings because they were only there to create space between outer and inner-walls. Suddenly, I’ve turned what was supposed to be a killzone into a new borough. Or I’m building massively expensive advanced buildings that provide tons of resources or new technologies… except they’re so costly that I can barely afford to take advantage of any of their improvements. I end up with unbuilt plans for steel barricades and electric fencing, but in the meantime, I’ve basically surrounded my city with plywood.

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Or I will lavish a vulnerable choke point with care and attention: Pouring resources into creating a deathtrap for the next horde that strikes at that point… only to for the next horde to come from a completely different direction, hitting a position that I somehow overlooked since the first fortifications went up at the start of the game.

And like, I shouldn’t have to learn this the hard way. This is basically why France’s failed Maginot Line became a modern parable: You prepare for a threat you’ve already faced, expecting it to behave in the exact fashion you’ve predicted, and then something different shows up that exploits the opportunity cost of all those self-indulgent pet projects. The under-baked defenses that got short-shrift while you were building your extremely local super-fortress.

In my earliest games, a lot that happened in They Are Billions was a surprise to me as I learned how the game worked. In my last several games, as my city turns into a graveyard, I am not seeing anything new. I’m just seeing evidence that I’ve made the same mistake as before and—for some reason—I keep growing unable to catch myself making them again.

I think it goes back to why They Are Billions is so compelling. A large part of this game is about developing a sense of psychological security. You survive a horde and then life returns to normal. The memory of your last close call fades away and you start trusting that the walls will hold the next time. You scale your preparations based on what you’ve seen before, even though you know on some level that the next time is going to be several times worse.

They Are Billions is an RTS at heart. You need to be using all the tools you’re given. Soldiers should be venturing out into the wilderness to thin the herds of infected before they reach dangerous numbers. You can’t just trust that fully manned towers suffice for defense; you need mobile forces ready to rush to the point of danger. You need to be racing up the tech and economy ladders without bottlenecking your development, because it’s the only way to build a base that’s efficient enough to defend itself. You need to do all of this because walls alone are never enough, but They Are Billions is designed and paced in such a way that it sometimes lets you forget what you're really up against.

So your urgency dwindles and your lags, because you have your walls and they are holding—untested—for now. It’s easy to stop venturing beyond of them because the threats are growing out there in the darkness, and if you confront them head-on you’ll also have to confront the fact that the water is rising higher and faster than you anticipated, and the emergency is in fact already here. But if you stay inside and keep making small improvements, you can convince yourself that you’ve thought of everything, that you’re safe and—if you’re not—you’ll still have time to fix things.

They Are Billions is a fun game because it lets you play around with the stuff of RTS fantasy: perfect clockwork defenses around efficiently-utilized resources that need never run out. But I think it might be a great game because in the space between its crises, it becomes a game about the lies of complacency in which we entrap ourselves. Its zombie hordes and teetering walls become metaphors for all the other things we let slide: the days we skip out on doing anything for our health, the projects deferred, the calls to loved ones we don’t return.

And I get it: we can’t survive in constant “emergency mode”. We need to tell ourselves that the small choices don’t add up to big existential decisions. But eventually they do.