Like an onion, or a baklava, SteamWorld Build has layers. What at first seems like a pretty straightforward city builder just keeps expanding and opening up as you play. After four hours with an early preview build, I'm still left wondering exactly how deep this rabbit hole goes.

Part of the varied and frequently excellent SteamWorld series, Build is the first entry not to be developed by the original team at Image & Form—but with its cute, steam-powered robot protagonists and creative genre experimentation, it fits Yono all app effortlessly into the canon.

You're charged, as you'd expect, with building a city—not just for robots to live in, but to support the greater mission of discovering ancient technology buried beneath the earth. The goal is to find enough parts to build a rocket ship and escape the planet, though why exactly your metal pals are so keen to leave, I'm not sure. It seems pretty nice around here to me. 

The basics of building are very simple—you plonk down houses to get workers, and then connect those houses to various services to make them happy. These services require their own resources, with simple production chains to manage—harvesting wood with one building to refine it into charcoal with another, for example.

Happy houses attract more workers, and once maxed out, they can be upgraded to a different type—first engineers, then aristocrats, then scientists. Importantly, though, these worker upgrades aren't simply better—higher tier robots unlock access to new buildings and resources, but they won't work at your existing ones, ensuring you always need to keep a balance in your population. 

But that's just what's happening above ground. Early on in the game, you unlock a mine—a full underground map you can switch to at any time. Well, you've got to do some digging if you want to find that ancient tech, right? Down here, things work differently—it's a Dungeon Keeper-esque management sim where you dig out rooms rather than plonk down buildings, and explore in search of rare resources and treasures. 

It's a wonderfully clever trick, basically giving you two linked games to play simultaneously. Keeping all your plates spinning in both places ensures you never run into that boring state of simply waiting for new resources to roll in—there's always something to be fiddling with, whether it's rearranging your residential district above, or sending your miners in search of a hidden door switch below. Though the two layers work quite differently, they're cleverly connected—resources found underground are sent up to the city to fuel further expansion, allowing you to create factories and refineries that produce elements you need to keep mining further out. 

But just as I'm starting to think I've got both layers running in harmony… I discover another mine shaft underground, leading to Yono all app an even deeper level. It turns out that, like SteamWorld Dig, Build is based around descending ever further through underground biomes, and I've just shovelled my way down into the mushroom-infested swamplands. 

Pest control

Not only am I now running three layers of settlement simultaneously, but this new biome has its own distinct challenges. Along with its unique new resources, it's also infested with insect-like enemies, and suddenly for the first time my bots are drawn into full-on battles. Fights are automated, any guards you've recruited scurrying wherever they're needed, but you can place various turrets and defences to keep key areas safe. When I start trying to excavate the area's ancient tech, suddenly I'm in the middle of a tower defence sequence, with swarms of bugs throwing themselves at my miners as grenade turrets and flame traps pop off all around them. 

All the while, my city's economy grows bigger and more complex, and I'm having to manage a trading system via a regularly arriving train to make sure I'm clearing out abundant resources and buying up rarer ones. Discovered technology can be slotted in to buildings to grant them powerful bonuses, but it's rare enough to make every choice difficult—do I need to speed up my sheet metal production so I can build underground bridges for my miners, or would it make more sense to get purified water flowing faster so I can make more workers happy? 

Needless to say, as I'm experimenting to find the answers to questions like these, I discover another mine shaft—there's yet Yono all app another level to explore… Given I'm less than four hours in, I have to assume there are then many more below that, each adding even more moving parts to the Rube Goldberg machine that is my settlement.

It definitely teeters on the edge of becoming overwhelming, but the game does a great job of keeping things accessible and clear, and the consequences of something going wrong are mild enough to ensure that things never get too stressful. What all these layers do is ensure that you can't rest on your laurels—there's always something new to do, and a whole handful of projects to work on, and if one part of your city seems to have peaked for the moment, that's just your cue to go back to messing with another. 

If it's anything like previous SteamWorld games, Build probably isn't super long—but given all I've discovered in just four hours, I can't wait to see what surprises it still has in store for me deeper in the earth. There's a more limited demo out now if you want to check it out for yourself—but believe me when I say, it only scratches the surface.