This is an awesome game! There are several things that could make it better though:
-Level up system (more attack for more kills, suppose more definitions are added or something)
-More augmentation options (upgrade attack, speed, ect. alone, not all at once)
-More money for higher levels (risk 11 adware gives more money than risk 1)
-Branching shield options (Firewall = SpeedWall, Firewall V2, SplashWall)
-Less attacking power for hijackers (get kinda annoying :P)
If those were added, this would be a groundbreaking tower defense game. You got my five, bud!