Some suggestions for "Starcraftish" elements:
* Normal - as the name implies
* Psionic - perhaps towers using energy to attack?
* Biological - concept of growth, have the towers grow more powerful over time by some means (e.g. by killing, damaging, time)
* Mechanical - maybe make these towers capable of linking together to achieve different effects
These are off the top of my head. Feel free to take any of these and twist them to fit your needs.