Defense Grid is an indie Tower Defense game by Hidden Path. Having played through the entire campaign I have to say that Defense is grid is a good solid albeit uninnovative tower defense game. I have game as well as the extra DLC maps but I didn't really play the extra maps too much.
There are probably two notable innovations:
3D maps- The maps are 3D in nature where towers on higher levels can shoot at lower level and paths can twist over or under other places. It's not six degrees of freedom as some reviews seem to imply but it does contain more freedom than other 2D tower defense games.
Cores and Resources - You have cores which represent the enemy's goal. The more cores you have left, the faster your resources regenerate. Regeneration of resources also increases the more resources you have. This creates a nice risk/reward dynamic where if you spend too much resources now trying to defend you can make the game harder later in the level but on the other hand, if you don't spend the resources now you might not last the entire level.
There are a couple of minor issues with th game design which I found while playing:
Key board shortuts all over the place- Why place L for "Fire orbital laser" and U for "Upgrade tower"? It's as if we haven't learnt anything from the last 20 years of gaming and still place keys all over the place.
Difficulty increases exponentially all of a sudden- Most of the earlier maps are fairly straight forward and then the difficulty suddenly jumps as the 3D aspects of the game suddenly become much more prominent. The expansion maps are also very tough and they are much more open-ended making the jump from the main game to them hard.
Lack of guide lights- As the maps become more open-endeded it becomes exponentially harder to know the path enemies will take to get to the cores. Especially for maps where there are various levels.Its strange then that there aren't guiding lights showing you the route of the enemies.
Well balanced but could use more special towers or special powers - Nothing you haven't seen before. You have your standard gun towers, flame throwers, artillary, slow-down towers, slow enemies, fast enemies and shielded enemies. Absolutely nothing stands out as unique about this game aside from the polished graphics and gameplay.
Overall, if you enjoy Tower Defenses, you'll like Defense Grid. You're paying and getting exactly what you expect a solid very polished game with good graphics and well-rounded gameplay and a variety of good game modes. If you prefer an equally solid game but with at least some innovation you'll probably prefer playing a game like Revenge of the Titans.