Last time we checked out Star Defense was late March. Since then publisher ngmoco has been gearing up for the game's release by fine tuning the mechanics of this 3D take on the tower defence genre.

Now we're just days from the game blasting off to the App Store and an updated build has been draining the battery on our iPhone. We've been particularly competitive as part of the May Day Twitter Challenge, but the result is that our enthusiasm for this stellar title has been sent into orbit.

Challenge mode embeds you on one of the game's eight planets for the purpose of defending a lucrative mine from a endless waves of S'rath forces. Enemies file out of a spaceship on one end of the sphere and weave along a set path in an effort to reach your mine at the other end. Your goal is survival: as the mine only carries 10 shield points, which is to say only that number of enemies can make it past your defences before it's game over.

You've given five different weapons with which to beat back the alien invaders - all turrets. Plunking down a basic bullet-spraying Gauss Turret will cost you $25. Kill a few enemies and money is added to you account, allowing you to buy the more expensive armaments like the fiery Neo-Plasma Blaster, the DX-3 Cannon which capable of slowing down more units, the electric Phase Coil, and big mama Quantum Launcher.

Tactics and overall strategy both play their own role. Since money is limited, you must deploy your defence to get the most bang for your buck. Additionally, strategic turret placements can help squeeze extra kills from your weapons. Enduring more than a couple dozens waves is a matter of prudent planning and not haphazard turret placement.

That said, your defences must be flexible to deal with a variety of S'rath foes. Gauss Turrets will suffice when dealing with low-level Probes and Swarms, but larger enemies like the spider-like Viceroys and bullet-resistant Quigons require bigger bangs. As you develop your defensive line beyond an initial smattering of cheap Gauss Turrets, you want to balance new units with upgrades to existing armaments.

Star Defense keeps things simple with only a handful of weapons, though longterm this could water down the strategic side of play. For example, it seems easy to pass over Neo-Plasma Blasters and Phase Coils entirely and build a line of Quantum Cannons. Enemies that possess a resistance to bullets will succumb to cannon fire, which minimises the need to plop down elemental blasters and coils.

Yet given that we've only had a chance to play on one planet, this balance could differ in the campaign and on other levels. Even if it doesn't, Star Defense still offers a challenging level of strategic play.

We'll have a full review of the game when it arrives on the App Store later this month.