Defend against the robo-bugs that are trying to take over your base. Unlock and upgrade new areas and special weapons. Collect enough exp points to get more powerful weapons.
Eliminate the invading force intent on taking Space Station Xorbe in Earth's orbit. If the station is captured then it'll become a staging ground for the invasion of Earth itself. You got a huge arsenal of weapons and defensive mechanisms to stop them.
Buy troops and tanks to place around the smuggling route. You have to stop the notorious drug trafficker that goes by the alias of 'Asian White Pearl'.
Build towers and survive through 51 hardcore waves of aliens that heal themselves, haste other enemies, split into more enemies and freeze your towers.