Tower defense games are great. Tired and done to death, sure, but the good ones are really fantastic. Anomaly Warzone Earth and the sequel Anomaly Korea are two of the better entries in the genre, having the distinguishing factor of playing role reversal. In these, you're not building towers, you're trying to slip past them. Now, both titles are half off on the Play Store, each costing a mere $2.
Normally, each game individually is priced at $4 and now you can get both for the same price.