Space Station: Frontier is the latest Android release form developer Origin 8, who also brought us the excellent Sentinel 3: Homeworld. This is a game with many similarities to Sentinel 3, and the same excellent production values in terms of graphics, sound, gameplay, and content.
This is a stranger take on tower defense which reminds me very much of Asteroid Defense, at least at the basic concept level. You start with one central space station, and you build structures by expanding outward from that starting point. That's pretty much where the similarities end, though. The way you build is by creating paths of power nodes which are required to build functioning towers. Everything is powered by your main base, and the amount of juice you use is a central game mechanic. You can build mining structures next to crystal asteroids to gain resources, and the rest of the towers are focused on defense and power storage. Running out of power while an enemy attacks pretty much leaves you dead in the water, so it's important to build and use your EMP blast intelligently.
There's a great variety of towers and enemies which all counteract each other in different ways. It can feel like a big round-a-bout game of "rock, paper, scissors" at times. The Battleships will usually stop out of range of your basic laser turrets, for example, while you'll need an interceptor tower to counter Destroyer attacks. The campaign is kind of like a really long tutorial and features a pretty basic sci-fi story. As you play, you'll be rewarded with credits to spend at the shipyard, and you're expected to purchase these towers once they appear in the campaign. You don't always have enough to keep up though, and you'll likely have to farm credits in Survival Mode just to keep up with expectations. It's a slow and annoying process. Along with Survival Mode, you can mix things up with the additional "Mining" and "Protect" game modes. Like Sentinel 3, your credits and what upgrades you purchase from the shipyard are universal across all modes. There are tons and tons of upgrades to purchase and it will likely take a long time to save up.
The graphics look pretty good normally, but there's a special HD mode and layout if you're playing on a tablet device like I am, and it looks just fantastic. The music and sound design are well done, and there is a ton of content here. There's also a handy help section outlining all the complexities of the game as well as hints and tips. Be smart, unlike me, and look at this section first. OpenFeint achievements and leaderboards are supported as well. Even though it can be frustrating and a very 'slow burn' kind of title, you're getting a lot for only two dollars, and I recommend it.