There are three types of towers that you can build:
Junk Tower - It shoots out clumps of space junk collected over the years around the space colonies. It affects only one enemy and it has 15% chance of miss. It shoots at a fairly quick rate but it has a small shooting range.
Plasma Tower - This tower shoots balls of plasma matter and liquid gas that vaporizes in contact with other matter, causing damage on everything in a certain radius. It has somewhat slower shooting rate but it affects multiple enemies and has larger shooting range.
Rocket Tower - It shoots out space homing missiles when an enemy gets into its shooting range. Rockets navigate towards the enemy with a 100% chance of hit. If an enemy dies before impact, rocket detects that and changes its target to the nearest enemy. This tower has the same shooting rate as the junk tower but it has a much larger shooting range.