A very nice start, but there's several things you could do to improve the game significantly -
1. Make it more interesting - it's actually OK for 30 seconds, but then it just becomes repetetive.
2. The enemy shots are too regular, too accurate and too fast. I can avoid them, but it's no fun
3. Never, ever, ever have enemies spawning behind you. Always give the player a chance to react to an enemy before it rams you.
4. When the game first started, I was avoiding the rocks and trying to collect the red things (Yes, like 99.9% of other people I didn;t read the ReadMe file until after). It's quite odd that the rocks don't kill/harm you. It would help improve realism if the rocks were different sizes and moved and rotated at differng speeds too.
5. Increase variety of alien movement, or increase the varities of aliens in each level. Having multiple types of enemies means the player has to think about his next move and how to deal with them and thusly makes the game more interesting. If all enemies are the same, then that thought process is non-existant and boredom sets in fast.
6. Sound. Sfx and music are incredibly important. Even simple whizzes, bangs and pops can enhance the atmosphere of a game, even without music.
