When you're sprinting through the lush, overgrown ruins of Colorado in Horizon Zero Dawn, the last thing you're thinking about is a grey box. You see the rusted ribs of a skyscraper. You see a Thunderjaw’s mechanical plates shimmering. But underneath all that visual fidelity—the grass, the lighting, the 4K textures—there is a skeleton. Game developers call it a blockmesh. Honestly, it's the most important part of the game that you will never officially see.
It’s basic. It’s ugly. And it’s the reason the game feels so good to play.
A blockmesh is essentially a low-fidelity 3D draft of a level. Think of it like a sketch before a painting. Guerilla Games didn't just start by placing beautiful trees and robot dinosaurs everywhere. They started with cubes. Thousands of them. If the Horizon Zero Dawn blockmesh phase hadn't been perfect, the entire "tactical hunting" loop would have collapsed.
The invisible bones of Meridian
Level design is hard. Like, really hard. You have to balance player guidance, combat arenas, and "the golden path"—the route the player is supposed to follow. During the development of Horizon Zero Dawn, the team at Guerrilla had to transition from the linear hallways of Killzone to a massive, sprawling open world. This was a terrifying jump. To manage it, they relied on a "whitebox" or blockmesh approach to test if a space was even fun before an artist spent three weeks making a rock look realistic.
Take the city of Meridian. It's a vertical marvel built on a mesa. In the blockmesh phase, Meridian wasn't a soaring city of stone and wood; it was a collection of grey platforms and ramps. Developers used these simple shapes to answer one question: How long does it take Aloy to run from the front gate to the palace? If it took too long, they moved the boxes closer. If the player got lost, they added a big "landmark" box to draw the eye.
It’s about metrics.
Aloy has a specific jump height. She has a specific mantle reach. She has a specific sprint speed. The Horizon Zero Dawn blockmesh had to be built precisely to these numbers. If a ledge was 2.1 meters high in the blockmesh but Aloy could only jump 2.0 meters, the level was broken. You fix that with grey boxes, not with finished assets. Why? Because deleting a cube takes one second. Redesigning a high-poly stone bridge takes three days.
Why the robots need grey boxes too
Combat in Horizon is a dance. You aren't just mashing buttons; you're using the environment. You're hiding in tall grass, ducking behind rocks, and climbing ridges to get a better angle on a Bellowback’s cargo sack.
This is where the blockmesh becomes a mechanical requirement.
During playtests, the designers would drop a Sawtooth into a grey-box arena. They needed to see if the "boxes" provided enough cover. Was the arena too cramped? Did the AI pathfinding get stuck on a 90-degree corner? By using a clean Horizon Zero Dawn blockmesh, the programmers could see the AI's "navmesh"—the invisible floor the robots walk on—without any visual clutter. It’s pure logic. If the robot can’t navigate a room made of cubes, it definitely won't be able to navigate a jungle.
The scale of the "Cauldrons"
The Cauldrons are probably the best example of blockmeshing done right. These are the sci-fi, subterranean dungeons where you learn to override machines. They are tight, platforming-heavy, and full of moving parts.
- Initial layout: Designers place large cubes to define the "rooms."
- Flow check: Can the player see the exit?
- Hazard placement: Where do the sparks fly? Where do the platforms move?
- Combat pass: Adding cover boxes for the inevitable fight at the end.
If you ever find a way to glitch out of the map in Horizon Zero Dawn—and people do—you might catch a glimpse of these primitive shapes lingering in the distance or hidden behind "Hero Assets." These are the remnants of the design phase.
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Mistakes designers make with blockmeshing
People think you just throw some boxes down and call it a day. Nope. The biggest mistake is "over-detailing" too early. If a designer starts adding little decorative boxes to represent furniture before they’ve even confirmed the room size feels right, they're wasting time.
Another issue is scale creep. In an open world like Horizon, it's easy to make a valley too big. You think, "Hey, this looks epic!" Then you play it, and you realize you’ve been running for four minutes without seeing a single machine or a collectible. The blockmesh phase is where you catch that. You shrink the boxes. You tighten the experience. You make sure the "fun density" is high.
Guerrilla Games used a proprietary engine called Decima. This engine is a beast. It allows for incredibly fast iteration, meaning a designer could tweak the Horizon Zero Dawn blockmesh and see the results almost instantly. This tight feedback loop is why the game feels so polished. It wasn’t an accident. It was thousands of hours of looking at grey cubes and saying, "This isn't quite right yet."
The transition from grey to green
Eventually, the blockmesh has to go away. This is the "Art Pass." Environment artists take the grey boxes and replace them with "modular kits." A long rectangular box becomes a fallen log. A tall cylinder becomes a crumbling pillar.
But here is the trick: the collision stays the same.
When you see Aloy’s hand perfectly grip a wooden beam, she isn't actually gripping the wood. She is gripping the invisible, perfectly square collision box that was defined during the blockmesh stage. The wood is just a "skin." This is why sometimes in games you might see a character's hand hovering slightly above a surface or clipping through a rock. It means the visual model doesn't perfectly match the underlying blockmesh.
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In Horizon Zero Dawn, the alignment is remarkably tight. This is a testament to the communication between the design team (who built the boxes) and the art team (who built the trees).
How to use this knowledge for your own projects
If you're a budding level designer or just a curious fan, looking at the Horizon Zero Dawn blockmesh philosophy offers a few real-world takeaways.
- Function over form, always. If it doesn't work as a grey box, it won't work as a masterpiece.
- Respect the metrics. Know your character's jump height. Build your world to fit that number, not the other way around.
- Iterate fast. Don't fall in love with your first layout. Be ready to delete half your work if the "flow" feels off.
- Sightlines matter. Use big blocks to hide things from the player, then reveal them for a "wow" moment.
To really see this in action, I recommend looking up the "GDC" (Game Developers Conference) talks by Guerrilla Games staff. They've occasionally shown behind-the-scenes footage of their development environments. Seeing a Thunderjaw stomp through a world of grey cubes is eye-opening. It strips away the magic and shows you the math and logic underneath.
Next time you're climbing a Tallneck, try to imagine it as a series of simple cylinders and cubes. It doesn't ruin the immersion; it actually makes you appreciate the engineering even more. The world isn't just "there." It was measured, tested, and blocked out with obsessive detail long before the first leaf was ever rendered.
Start your own design by focusing on the "Golden Loop." Determine the one thing your player will do most—in Horizon, it’s movement and tactical aiming—and build a simple blockmesh arena specifically to test those two things. Only once that feels "sticky" and responsive should you even think about the textures. Most failed indie projects happen because the dev spent six months on a character's hair physics and zero days on the blockmesh flow. Don't be that dev. Build the bones first.