The Distance Seeder is a small defense tower with high range but little damage. He is cheap to build and is the best defense when you have no money.
The Solar Intensifier is an air defense position. This AA Tower is your last defense against flying enemies but the Solar Intensifier can only attack enemies in the air.
The Lightning Generator is one of the heaviest defenses. He produces an arc that burns almost everything. The disadvantage is that he has to recharge before he can shoot.