If you make a platform, they will come. Within minutes of releasing the specifications of a new piece of hardware, half a dozen "Tower Defense" games will have begun their development for the fledgling platform. The DSi is no exception - as a relatively new console it may have a smallish library, but that library already includes Starship Patrol, a tower defense game set in space.
For those who don't know what a tower defense game is, it breaks down something like this: place turrets in fixed locations to defeat regular waves of enemies. It's like a stripped down Real Time Strategy game, removing the complexity from the mechanics but retaining the strategy.
Starship Patrol takes this concept and deploys it in space, tasking the player with defending their array of space ships on the bottom screen from waves of enemy fighters that assemble (Galaga-style) in the top screen.
Destroying fighters spawns energy balls which slowly fall to the bottom of the screen - you'll need to collect these with your stylus to provide currency to buy more turrets. They also have a chance to spawn a powerup, which you can install in a turret to increase it's speed, power or range.
Putting the right powerup in the right place is almost as important as choosing where to put your turrets - you can't remove a powerup (although you can install a new one to replace the old) and you have to pay to destroy a turret you've already built.
The layout and combination of ships to defend varies from level to level, which in turn changes the paths that are available for the incoming waves. Unlike many tower defense games the incoming enemies don't necessarily take the same path every time. This one twist to the mechanic, combined with the low rate of generating the resources necessary to build your defensive turrets, contributes to the bulk of the difficulty - which is high.
You'll find yourself spending all of your resources just barely keeping the first few waves at bay only to find a subsequent wave attack from a completely different direction which is completely undefended. Fortunately each level follows the same pattern so you have the opportunity to learn the sequence and plot a careful strategy that will allow you to survive it. Unfortunately, these levels are quite long which means that as you'll typically fail near the end, actually defeating a level after a number of attempts can mean a long time in one place.
Visually starship patrol has a wonderful style, reminiscent of architectural doodles on graph paper or a maths book. It scales well to the DSi XL's massive screens and achieves strong enemy silhouettes without requiring a bevy of expensive artists to generate them or dozens of megabytes to download them. Basically it's the perfect combination of visuals for a downloadable strategy game.
Ultimately if you're into tower defense games and have a hankering to play one on your DSi, this is a no-brainer. At 500 points ($15) it's relatively inexpensive compared to the premium DSiWare titles (normally around 800 points, or $24). And Starship Patrol's feature set and visuals allow it to stand side by side with any of those. It's a bit hard perhaps, but there's a lot of depth here, and the skills you learn will genuinely improve your ability to blitz through levels you previously struggled with. Highly recommended.