Tower defence games are a peculiarly compelling niche within puzzle games. Perhaps it's because you are still ultimately shooting stuff that I find them so entertaining, but I suspect it's a little deeper than that: they are a tinkerer's delight, a kind of fine-tuning of instruments and setups to discover what works best. It's the classic pastime of toying with a machine, carefully learn how it works, and figuring out how it could be improved. While I'm playing these I'm a boffin in a shed at the end of the garden, searching for the best solutions to pointless problems. Defense Grid: The Awakening is one of the best tower defence games (yes, I'm spelling it with a c, I'm an Englishman) I've ever played.
So: bads come in to try and steal your resources, and you must build a grid of towers to stop them. Most towers shoot, but they all have different properties. Some might be anti-air, some slow down opponents, some bombard from afar. Getting the mix right will decide whether you win or lose. That's tower defence, and it's a fun time.
Defense Grid is beautifully presented and absolutely solid in every way. There's little innovation here, but the improvements on the basic idea of tower defence that the team have made are all implemented comprehensibly and sensibly. Towers can be upgraded, for example, and you soon learn the value of an upgraded tower over those that have simply been built up in their basic mode.
Visually it's rather fun, with each map being a ruined sci-fi grid, each in a different location - on a dusty planet, amid ice, or lava. What scene setting and explanation the game needs is provided by the voice of the "AI" your mentor and guide through the various levels. As the game expands and brings in new towers and power ups, so the AI explains to you what needs to be done. For such a functional role, he's actually fairly well acted, and quite entertaining.
The game itself is ludicrously compelling. Even the really tough maps have had me repeat and repeat, rather than quit to do something else - no mean feat with an attention span as gnat-like as mine. I've forgotten the last time I was so hooked - to the point where I've been scrounging minutes here and there, between work and domestic chores, just to get another round in. I thoroughly recommend this game, and I'm certain lots of other folks will have as much fun with it as I have. It's a little on the expensive side, sadly, and should clearly be aiming for the $10 bracket.