Ghost Hacker is one of a multitude of tower defense games that I've played.
It's of the "non-mazing" variety, i.e. the enemy moves on a preset path (which, I would suppose, makes it much easier to program). These are my less favored type of tower defense, although I still enjoy them.
What sets this apart from the average tower defense game is the tower customization system. Each tower has two or three "hard points" on it that you can attach an upgrade to. Whereas in most tower defense games, an upgrade is just a perk, here they're half the point or more of the game. Each one grants a special benefit. Each tower is also unique in where it can be placed and what it does. A lot of gameplay is figuring out what combinations are coolest or most effective. (Hint: Splash damage modifiers stack.)
Enemies in a game like this can also make the difference between being generic and interesting; the enemies here are definitely unique and interesting. That said, they're also total (expletive deleted) to deal with.
I think Ghost Hacker is a very interesting game, although it's a bit of a pain to play at times. I can recommend trying it.