I think it's very impressive that you have got 3 different styles of gameplay here, with the fighting waves of enemies, upgrading the space station and flying around in the spaceship to gather materials! I think using the materials to upgrade the base is a good use of the theme, I don't think I've seen something like that here yet!
I like the art style, with the distinct buildings in the space station and I like the addition of planets around in space, that's really well done for the time limit it was made in! I like the ui of upgrading the buildings too.
Good Job!