This is a fun concept, and a good use of the theme. But there are some things that can be improved here:
The enemies spawn right next to where my units spawn, and without warning. If there was like a ship orbiting the planet spawning the enemies where I can see them and not on top of my units that would be a lot more fun
When all the characters are on top of each other it's impossible to click on the one I want. If they had some sort of collision keeping them separate they would be easier to control