I haven't played many tower defense games, but it is clear to me that "Defense Grid: The Awakening" is a high quality example of the genre and a very good game in general. I recommended this game to a friend of mine. He currently has 101.5 in-game hours.
Be warned: It is addictive! This game will have you chasing Gold, Silver, and Bronze medals and the 87 achievements. I paid full price for this way back when (about $20) and it has been worth every penny. At it's current price of $9.99 you cannot go wrong. There is a reason that user reviews are "overwhelmingly positive." It's the kind of game you might uninstall after completing, only to eventually reinstall to complete more challenges.
The levels are presented in a nicely tuned progression of difficulty. The early levels are a great introduction for anyone new to tower defense. Aliens enter the screen and follow both prescribed and influenced pathways, along which you erect towers with various functions to destroy them.
The goal of the aliens is to steal power cores located at a central location and then escape the levels with them, usually retracing their steps although this can also be influence by the design of your defense. If all the power cores are stolen, the level is failed. Early on it was unacceptable to me to finish a level with even one missing power core. Striving for perfection is an enjoyable though sometimes maddening struggle, but satisfying when completed.
The presentation is very polished with a healthy variety of interesting aliens, of both walking and flying types. You may also zoom in on the action to get a more detailed view of the fight. The voicework, sound effects, and music are all satisfying. You have a "companion" in the game in the form of a voice (an artificial intelligence as I remember), sometimes helpful and sometimes not as much.
There is also a good variety of animated towers and in the levels where many different types are working together, the screen nearly resembles a fireworks display. It is VERY satisfying to find an effective defense and then sit back and watch the mayhem. Players also have access to a laser beam fired from space to wipe out an area of aliens. It may only be used a handful of times on any one level and takes time to recharge. This is very helpful for vaporizing aliens that have gotten past the defenses and are escaping with power cores.
I have no complaints with the controls at all. I was never fighting the UI. There is also a nice feature of the UI that gives a bit of warning regarding the types of aliens that are on the way. It's like a news channel "ticker tape" with icons for each type of alien. Also aliens do not attack towers in this game.
The DLC for this game is also worth the price as well. The four map packs are a good value for $.99 each. There is even a Portal themed DLC, "You Monster", featuring GLaDOS, who hijacks your UI's AI.