Year 2281. The first habitable planet is found... it's colonization is made through the portals. It turns out that humans are not the only sentient race in the universe... and the other sentience is not friendly at all!
Place towers using your mouse to prevent the creeps from passing through the portal without completely blocking their path. Upgrade your towers to increase their rate of fire, range, and damage. You also have special abilities, but you can use them when fully charged. Don't forget to spend your stars on skill upgrades between missions!