Factorio Biter Defense Guide: Turrets, Walls, and Bots

Biter defense is the thing most new Factorio players get wrong, and it's usually not the fighting part. It's the setup. You throw down a line of gun turrets, forget to feed them ammo, and then wonder why half your furnace array is gone by morning. The good news is that defense in this game is mostly a plumbing problem. Get pollution under control, build walls that make sense, and let robots handle the boring upkeep, and you can mostly stop thinking about biters entirely.

Understanding Pollution and Biter Evolution

Everything about biters runs off one hidden number called the evolution factor. It sits between 0 and 1, and it climbs from three separate sources: time passing, pollution your machines produce, and spawners you destroy. You can check your exact number and the breakdown any time by typing /evolution into the console.

The thresholds matter more than the number itself. Medium biters start showing up around 0.2 evolution, spitters join around 0.25, big biters arrive near 0.5, and behemoths show their faces around 0.9. On default map settings with zero pollution and zero nest clearing, hitting 0.2 takes roughly 17 and a half hours of pure clock time. That's a long time. In practice, pollution gets you there way faster.

What actually pushes evolution up

Here's the part that trips people up. Evolution counts pollution your machinery produces, not pollution that reaches the nests. You can plant a forest, build a ring of trees, whatever you like. The cloud gets smaller and you get attacked less often, but the evolution number keeps climbing at the same speed anyway.

The three inputs aren't weighted equally either. Community testing landed on a handy rule of thumb: about 2,222 pollution produced, one spawner killed, and roughly 8 minutes 20 seconds of game time each add roughly the same amount of evolution. Time is the slow drip. Pollution is the firehose.

Killing spawners gets less punishing as you go, since each increase gets scaled down by a factor based on how far along you already are. Clearing a nest at the start of a run might bump evolution by 0.002, while doing the same at 0.5 evolution adds about 0.0005. So nest clearing is expensive early and nearly free late. Worth remembering before you go on a rampage at hour two.

What the pollution cloud actually does

The cloud controls attacks, not evolution. When a chunk of land near a nest has more than 20 pollution in it, that spawner starts soaking pollution up and spending it on attack units. Each size costs the nest a fixed amount:

| Unit | Pollution to join an attack | | --- | --- | | Small biter | 4 | | Medium biter | 20 | | Big biter | 80 | | Behemoth biter | 400 | | Small spitter | 4 | | Medium spitter | 12 | | Big spitter | 30 | | Behemoth spitter | 200 |

Once the nest has absorbed enough, biters gather at a meeting point and launch the attack somewhere between 1 and 10 minutes later, with up to 2 extra minutes of waiting for stragglers. So a fat pollution cloud sitting on six nests means big frequent raids. A thin one means the occasional scouting party.

Cutting pollution at the source

Boilers are the usual early-game villain at 30 pollution per minute each when running flat out. A coal power setup with a dozen boilers is doing more biter baiting than your entire mining operation. Burner mining drills put out 12 per minute, electric drills 10, and a stone furnace only 2.

A few changes make a real dent. Swap coal power for solar or nuclear and the boiler problem disappears completely. Drop efficiency module 1s into your miners and furnaces for an 80 percent energy cut, which cuts their pollution by the same amount. Efficiency modules are the only way to reduce what a machine produces in the first place, so they're the strongest tool here. Trees help too, since they soak up cloud as it drifts, but that only buys you quieter attacks rather than slower evolution.

Wall and Turret Placement That Actually Holds

A stone wall has 350 health and solid resistances, including 80 percent acid resistance and 20 percent physical. That acid number matters a lot, because spitter puddles melt almost everything else in the game. Walls shrug them off. Five stone bricks apiece makes them about the cheapest thing you'll ever build in bulk.

Two mechanics decide whether your wall design works. First, walls only block melee. Spitter acid flies right over, so a wall alone does nothing against a wave of big spitters. Second, big and behemoth biters can reach about 1.5 tiles, which means they chew through a single wall layer and keep hitting whatever's directly behind it. Your turrets, usually.

The basic layout that works

Fix for the reach problem is simple. Either double up the walls or pull your turrets two tiles back from the line so the big ones can't touch them. A double wall is cheap insurance and saves a lot of repair packs.

The standard shape looks like this:

1. Two layers of stone wall out front
2. Flamethrower turrets right behind the wall, spread along the line
3. Gun or laser turrets another tile or two back
4. Roboports and power behind those, out of spit range
5. A belt or requester chest feeding ammunition to the gun line

Spacing matters more than people think. Gun turrets reach 18 tiles, laser turrets reach 24, flamethrowers reach out to 30 but have a minimum range of 6. Spread your turrets along the wall instead of clumping them at corners, and make sure their fire overlaps the ground in front of the wall rather than just the wall itself. You want things dying before they reach the stone.

Dragon's teeth

Dragon's teeth are those diagonal grids of single unconnected wall pillars you see in screenshots. The idea is that biters will path around obstacles rather than smash them, as long as a path exists. So instead of punching through your wall, they wander a zigzag through the teeth while your turrets cook them.

It works, with caveats. Big swarms still chew through if the maze annoys them enough, and one community observation worth keeping in mind is to avoid placing the teeth directly on chunk borders, since that seems to aggro biters onto the walls for no good reason. Whether that's a quirk you believe or not, the practical version is simple: leave a path, keep the teeth shallow, and don't expect them to replace guns. They just buy time.

Letting robots do the repairs

This is where the guide title earns its keep. Construction robots with repair packs will automatically fix anything damaged inside a roboport's construction zone, and a roboport's construction area is a big 110 by 110 tile square with a 50 by 50 logistics area inside it. Chain roboports along your wall so the green zones overlap, keep repair packs stocked inside them or in a nearby provider chest, and your wall becomes self-healing.

Robots are dumb about timing though. They'll fly out mid-battle and get eaten by acid puddles, which is a great way to burn through bots. Big spitter puddles deal 130 damage per second for 32 seconds, and behemoth puddles hit 360 per second. So give the repair packs a circuit condition or just accept some losses. A chest of spare walls and a few dozen spare bots near the perimeter keeps the whole thing running while you're off building trains.

Laser Turrets vs Gun Turrets vs Flamethrowers

There's no single winner here, which is kind of the point. Each turret does a different job, and the best defenses mix at least two of them.

The raw numbers from the wiki:

| Turret | Health | Range | Power or fuel | Notes | | --- | --- | --- | --- | --- | | Gun | 400 | 18 | Ammo belts | No power needed | | Laser | 1000 | 24 | 800 kJ per shot | Big power spikes | | Flamethrower | 1400 | 6 to 30 | 3 fluid per second | 120 degree arc |

Gun turrets

Cheap, available from the start, and they need zero electricity. An inserter keeps them fed, and inserters will only load 10 magazines per turret, so set up a proper belt line rather than hand-feeding. Their range is the shortest of the three at 18 tiles and their health is the lowest at 400, which makes them fragile against acid.

The catch is biter armor. Big biters have 8 flat physical damage reduction plus 10 percent physical resist, and behemoths sit at 12 flat and 10 percent. Regular bullets fall off hard against that. Piercing rounds hold on longer, and uranium rounds turn gun turrets into something genuinely scary late game. Physical damage research also stacks multiplicatively with your ammo bonus, so a heavily researched gun line with uranium ammo out-damages almost anything.

Laser turrets

The convenience pick. No ammo logistics at all, just wire them up. Range of 24 tiles beats gun turrets, and nothing on Nauvis resists laser damage. Biters have zero laser resistance across every size, which is a nice change from the bullet situation.

The problem is power. Each shot costs 800 kJ, and a wall of 30 lasers firing at once will brown out a marginal grid. When your power dips, your turrets slow down, and a slow turret wall is a dead turret wall. If you go heavy on lasers, build accumulator banks sized for a full attack, or keep steam turbines in reserve. They also cost a fair bit more than gun turrets to make and need steel, circuits, and batteries.

Flamethrower turrets

The best value in the game, most of the time. They run on crude, heavy, or light oil straight from a pipe at 3 fluid per second, and oil is basically free once your refinery is going. Light oil gives a 10 percent damage bonus over crude if you feel like being fancy about it.

What makes them nasty is the burn. Anything ignited takes 100 fire damage per second for 30 seconds, which works out to about 3,000 total damage. No biter or spitter has any fire resistance at all, so that number lands in full. A single flame turret can wipe a whole wave that a row of gun turrets would spend all their ammo on. Ground fire even intensifies the longer the turret keeps shooting at one spot, up to 6 times the base damage.

Two real downsides. They have a 120 degree firing arc, so they have to sit at corners and chokepoints rather than spraying in every direction. And they have a minimum range of 6 tiles, meaning anything that gets close is invisible to them. That's why you pair them with guns or lasers to finish off survivors.

The verdict

For most bases, flamethrowers do the killing, gun turrets handle what walks through the fire, and lasers cover spots where running a belt or a pipe is a pain. Outposts and mining patches far from home are natural laser territory since a power wire beats a whole ammo supply chain. If your power grid is shaky, lean guns and fire instead.

Using Artillery to Clear Nests Safely

Artillery changes the whole game. One turret reaches 224 tiles in automatic mode, which is 7 full chunks, and 560 tiles when you're manually targeting with the artillery remote. Nothing else in the game comes close to that reach.

What artillery does is kill nests without you being there. Automatic mode only targets enemy structures, spawners and worms, never the units themselves. Anything caught in the blast takes damage anyway. A shell one-shots every worm size up to behemoth, and it one-shots spawners up to roughly 0.72 evolution with no damage research. Past that you need artillery damage levels to keep the one-shot streak alive, since spawner health scales up with evolution.

The catch

Artillery starts fights. Every shell that lands on a nest sends a retaliation wave, and testing by the community pinned down the detail: the biters aggro on the position of the turret or wagon at the moment the shell explodes. They come for the artillery, not for your main base. That's usually fine, and it's honestly great, because it means the fight happens at a spot you chose and already fortified.

So the rule is never place naked artillery. A cluster of artillery turrets with a wall, flamethrowers, lasers, and roboport repair behind it will eat retaliation waves all day. Artillery sitting alone on a hill will be gone in ten minutes. The turret itself has 2,000 health and decent resistances, but a wave of big biters doesn't care about your resistances.

Keeping it fed

Artillery shells aren't free. Each one needs steel, plastic, explosives, and a circuit, plus a rocket part's worth of processing, so a shell factory is a real commitment if you're running multiple turrets. Set up a dedicated assembler line with a belt or a train feeding the turrets by inserter, same as gun turrets but with bigger magazines. Inserters can chain shells between artillery turrets too, which saves on belt spaghetti.

If you want mobile clearing power, the artillery wagon is the same weapon on a train car. Roll a defended train out to a new ore field, shell everything within range, then head home. A lot of players run a permanent artillery outpost train on a schedule that tours the perimeter and keeps nest regrowth in check.

Manual targeting with the remote is worth learning too. One click is one shell, you can fire at terrain you haven't even explored since the shell reveals the chunks it crosses, and the remote shows you how many loaded turrets are in range of your cursor. Great for picking off that one nest your radar just spotted.

Building a Perimeter Defense That Scales With Base Size

Early on you don't need a perimeter. You need a few turrets on the side of your base that has nests. A full wall around a small starter base is a waste of an hour. Defend the approaches that actually get attacked and ignore the rest until the map tells you otherwise.

Once pollution spreads though, attacks come from wherever the cloud touches a nest, so at some point encircling the whole factory becomes less work than patching holes. Two big decisions shape everything after that.

Use the terrain

Water is a free wall that biters can't cross and spitters can't meaningfully attack through. Lakes and coastlines cut your build cost enormously, and a base tucked against a big lake might only need half the perimeter of one sitting in open grass. Cliffs help too, since biters have to path around them unless they chew through. When you're picking where to expand, look at the map first and let the geography do a chunk of the work.

Build it tileable

The trick to a perimeter that grows with you is blueprints. Build one chunk-aligned segment, about 32 tiles wide so it matches the map grid, containing a double wall, a line of flamethrowers, gun or laser turrets, pipes, power, and a roboport. Save it, then stamp it end to end around your base. Now growing the perimeter is a copy-paste job instead of a rebuild, and damaged segments get replaced by robots automatically.

Chunk alignment has a second benefit. Pollution, biter pathing, and spawner behavior all run on chunk logic, so walls that line up with the grid tend to behave more predictably than ones that cut across it.

Keeping the whole thing alive

A big perimeter is really a logistics problem dressed up as a military one. The parts that fail aren't the turrets, they're the supplies.

• Roboport chain along the wall with overlapping construction zones
• Repair packs and spare walls in storage or provider chests within the logistics network
• A radar network so you see nests before they see you
• An ammunition belt or bot-based delivery for gun lines
• A light oil supply piped or trained out to every flamethrower
• Power coverage, with accumulators near laser clusters if your grid is tight

For distant outposts, a supply train that visits on a timer works better than trying to belt everything from home. Players who run artillery tours usually have that same train carry shells, walls, repair packs, and spare bots so the far defenses never run dry.

Circuit alerts are the last piece. Wire a wall segment or turret to a speaker and you get an audible warning when something takes damage, which beats finding out three hours later that a corner has been quietly gone since Tuesday. You can also wire gun turrets to close gates or trigger alerts when their ammo runs low.

The endpoint of all this is a base where biter defense runs itself. Pollution stays managed, the wall holds, robots patch the damage, and artillery keeps the nest count down before it becomes your problem. At that point the only time you think about biters is when you decide to go pick a fight.