Anomaly Defenders ( on Steam here: http://store.steampowered.com/app/294750 ) is the closing installment of the Anomaly series. The original incarnation of the Tower Offense sub-genre had players controlling humans fighting alien invaders. Now, the tables have turned. The human counterattack is underway and the alien homeworld is under threat. Defend the planet from the human scum in the final battle of the series.
DRM-free version (with lots of extra goodies including Steam key) can be grabbed from: https://gamesrepublic.com/bundle/strategy,anomaly-defenders-anomaly-2,368.html
More info on Facebook: https://www.facebook.com/anomalygame