Defense Grid's gameplay is deceptively simple. The player is allotted resources to build defensive towers, each with specific strengths and weaknesses. Towers are indestructible and never run out of ammunition. Players use them to repel waves of single-minded aliens that are looking to steal the power cores from the base. Killing aliens earns additional resources, which can in turn be used to build or upgrade towers. Aliens are generally averse to walking directly through a tower's energy field unless they are completely blocked from their goal and have no choice, so given enough resources, players can use their towers to set up elaborate mazes of destruction to keep enemies at bay.

Resources are scarce, and the maps can get pretty tricky, so players need to plan out how they're going to deal with the alien assault. Each tower upgrade costs double the amount of the previous upgrade of purchase cost, so fortifying key areas quickly becomes expensive. Upgrades also take the tower out of commission, leaving an area vulnerable for a short period. Lastly, these defensive towers can't shoot through one another, so players need to take line-of-sight into consideration when planning their defense. It all sounds tricky, but it all comes down to building the perfect defense system to keep aliens from hauling off with the power cores. Stolen cores can be recovered by taking out the aliens carrying them, and they'll slowly float back to reactor. However, aliens can still snatch them up mid transit. Oftentimes, larger units will pass their cores on to small, faster ones once they're destroyed, which can pass them off to other groups in-turn. Players will need to consider potential passing chains in addition to repelling aliens. Keeping cores at the power station charges up an orbital defense beam that will obliterate any alien caught in its blast, but players won't receive resources from their destruction.
This is like a sci-fi SimCity with an edge. Defense Grid skillfully combines a puzzle-game vibe with fast paced real-time strategy. It wasn't long before we become so engrossed in setting up the perfect defense that we completely lost track of time. There's no option to pause the game and look over the map, so players will either need plan quickly or experiment with multiple retries. Players can't controls what their defense towers shoot at. The computer does an excellent job of prioritizing targets, but we still would have liked some way to order towers to either spread out or concentrate fire on specific units. It's also difficult to see how well a specific tower type impacts a type of alien, apart from singling one out and examining it to check on its health stats or reading about it in the often vague alien encyclopedia. Aliens usually travel in huge groups that can draw fire away. A clear and easy way to see the alien's condition, like bleeding, sparks or different movement animations would be very useful. As is, the only way a player can tell whether a tower type works is if the alien can walk past it or not.

The game's twenty levels move swiftly. You'll soon find yourself in extremely challenging situations where hordes of aliens, armed with stealth and energy shields, come from multiple entry points to quickly make a bee-line for your power cores. Beating a stage unlocks different challenge modes such as beating the stage with a limited number of towers. There's also a mode called Grinder, where players need to hold out against 99 waves of aliens of increasing toughness. Replay value depends on how much you want to challenge yourself, because the main storyline is relatively short. It's too bad the game currently doesn't have a map editor, although there may be one in the future.

Defense Grid: The Awakening is one of those games that quickly gets under your skin with its "easy to learn, difficult to master" style. Soon, you'll be lying up awake a nights trying to figure out how best to use your resources to build an unbeatable defense.