Augmented reality toys have become a small but steady part of the gaming market, but augmented reality as a concept has always been hit or miss. If the companion app isn't a tacked-on bell and whistle, the toy itself often isn't much fun to play with. But Lego, which has a long history of blending tech into its traditional building sets, may have struck a decent balance with a new project called Fusion.

Fusion's premise isn't too different to that of similar toys: put an object on top of, or next to, or under a tablet, and a version of it will appear in a corresponding app. It begins with four fairly normal-looking sets, each based on a different Lego theme: race cars, a town, a medieval castle, and a resort (part of the girl-focused and frequently maligned Friends lineup.) But among the pieces is a small plastic base, each set's version emblazoned with a different pattern. Build on top of them, and your tablet's camera will be able to recognize and scan the resulting design, as long as it's not too big and adheres to some other rules. Whatever you make will end up being part of a mobile game.

Build a facade, and the app will turn it into a 3D building

In the Town Master set, for example, you build a house's facade on what looks like a piece of sidewalk. Start the app, point the camera at your creation, and you'll be asked to line up its base with a small box on the screen. Once you get it right, the app scans the bricks and matches them by color and shape, reconstructing a virtual counterpart. The app stretches it into a 3D model, and you can place it in a town-building game that's reminiscent of a highly simplified SimCity. In the beta version I tried, scanning success depended on good lighting and a few retakes, when the design came out with random black horns or missing bricks. It wasn't difficult, though, and it didn't require any special features like NFC. Once a building is scanned, you can check citizens' needs and assign it the role of a hospital, restaurant, "Segway store," or any number of other businesses and utilities.

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Granted, this isn't remotely approaching the level of SimCity or any number of resource management games, and all the sets are geared towards younger children. There's not a lot to do besides scanning and placing houses, collecting money, buying goods, and playing a few mini-games. But it's surprisingly fun. The apps for Town Master and the other sets are made by TT Games, the company behind the endless and fairly well-received lineup of Lego video games. As Lego design manager Ditte Bruun Pedersen told me, they're designed around the most crowd-pleasing mechanics: "tycoon" world-building, tower defense, and racing.

Unfortunately, Town Master was the only one of the games I got to try firsthand, and it wasn't in a finished state. I was able to watch a demo of the medieval Battle Towers, a combination of tower defense and, well, tower-building. Players use their base to make a series of facades that are stacked on top of each other in the game. These can be fortified, but they can also be damaged — and if they are, the player will have to quickly rearrange physical bricks to repair them. The resort combines town management with interior decoration. A racing game assigns stats to different pieces, letting you optimize your car. If you race with friends, you can look through their designs to find ideas. The whole thing is reminiscent of Crayola's scannable coloring books, but they're more reusable and a little more intuitive. The big downside is the single base, which limits how much you can build at once.

A universal scanner probably isn't in the cards

The obvious endpoint is a universal scanner that can pull any design into a game, like some kind of augmented reality Lego Second Life. The technology is limited right now, and so are these sets, but if Lego is accurately describing them, there's more flexibility than you might think. Each app is set up to recognize certain colors and shapes, but it will do its best to identify bricks from outside the set, finding the best fit in its existing list. You can't, for example, make a rubber wheel appear on your castle battlements. According to Pedersen, though, you might be able to hang one up and see it turn into a window or any other medieval piece that's roughly the same color and size.

Some other augmented reality systems have done well: Activision saw early success with Skylanders, the more recent Disney Infinity just added action figures from the Marvel universe, and Nintendo announced its own "Amiibo" NFC toys last week. Unlike all these, though, Lego Fusion doesn't scratch the collector's itch. Its designs seem designed to be ephemeral, getting put together, scanned, and taken apart on a whim. But that also expands the range of options, making it more interactive offline as well as online. And if it takes off, we'll get to start pondering the possibility of an endlessly recursive Lego Minecraft set.