You've probably played a million tower defense games, and "orbital shooters" aren't too uncommon either, but what about playing them at the same time? Artificial Defense combines a few different game types with some nerdy computer vernacular to create a distinctive mobile game, and you can give it a shot for free.
Your goal in Artificial Defense is to prevent the intrusion of this Tron-esque world by malicious programs. At your disposal are various defense towers and weapons that can be dropped on the battlefield from your vantage point high above. Using RAM earned from combat, you can upgrade your arsenal to do more damage. There's also an entire "conquer" mechanic in the game that requires you to send special units to retake parts of the system that have been corrupted.
Artificial Defense has a steep learning curve with multiple things to keep track of. It does come with a few tutorials that take you through all the game mechanics, though. You can play the first few levels for free, then the full version is about $3 with no additional in-app purchases.