The idea of dropping 20 bones on a tower defense title seems ridiculous when you consider how many free variations are available. Instead of a static screen with simplified 2D graphics, however, Defense Grid: The Awakening feels more like a real-time strategy game, complete with big maps to scroll across, and 3D structures and units. And it's not just the look – a huge portion of Defense Grid relies on your ability to tactically approach each level in order to protect your base from invading aliens who actually have a motive. Yet it doesn't force you to rack your brain in order to succeed until the later stages, allowing for an easy entry title for new players, while simultaneously challenging tower defense vets.

Like most defense games, the concept is simple: you need to build multiple attack towers to fend off invading forces that enter from Point A and try to exit at Point B. The better the layout, the easier defending your base will be. As opposed to the traditional "build anywhere" basis, Defense Grid restricts construction to specific platforms, which challenges you to appropriately place your structures. Furthermore, a list of variously-powered cannons, guns and lasers means you'll also need to consider which would best suit your resource spending at a given moment. Rather than stopping enemies from reaching a certain point, however, you're efforts go toward defending the shiny blue balls known as Cores that the invaders are attempting to steal. Acting as a power source for your bipolar AI guide, the English-accented narrator who powers your towers, accumulates interest on earned resources, and pushes the thin "Uh-oh, aliens!" story along.

It might sound like the game is convoluted, but it's all very easy to grasp. Complexity builds slowly, and Defense Grid does an excellent job of teaching you how to manage everything as you progress. To make things easier, your cursor is locked at center-screen. You can swiftly navigate the stage without losing track of where you are and it's incredibly accessible for anyone who hasn't dipped in to the genre before. The downside is that you aren't able to customize the controls, so if you want the perspective to stick while your cursor cruises around you're out of luck.

The same goes for the camera, which has three levels of zoom but no means of swivelling side to side. Another unfortunate result of the interface is that navigating menus isn't nearly as efficient as it should be. You'll navigate the rigid boxes with the mouse, and frequently select something you didn't mean to thanks to the lack of a cursor. Alternatively, you can use the arrow keys, but it slows you down and is more inconvenient than expected.

In spite of the clunky control, it's still very easy to use, and the action is slow enough to allow you plenty of time to scheme. Once you've solidified a game plan, you can watch your towers take down the invaders while upgrading units' attack range, power and fire rate by plunking down an exorbitant amount of cash, which is earned by turning your foes in to space-paste. Sacrificing more towers in favor of enhancing existing ones is a definite risk, and you'll find the results are typically advantageous in both regards. It becomes a question of context: what would best suit my defences right now?