A mix of TD gaming and FPS (Neither overpowered or neglected) a true balance of the mediums!
The enemies are all difficult in their own unique ways, the weapons and towers all have their own strengths and weaknesses that make none an "end all be all" for an entire base.
There's some neat Easter Eggs if you feel like exploring, the game universe is detailed and a pleasure to look at. The Achievements aren't ridiculous or "a cake walk". The game's visuals are fun and the gibbs come across nicely from dead enemies.
The only down side is that it was released too soon. There's only 3 maps at this time, the most basic of towers, and 2player coop. The good news is the Devs are making more maps, promised to add new towers and weapons, as well as promised to work on 4player multiplayer.
Again, if you love Tower Defense games and enjoy a good FPS, this is the best thing since "Desktop Tower Defense Pro" / "Balloon Tower Defense 4" / "Defense Grid: The Awakening".