I'm sorry to bump this old thread, but while revamping my website (http://xarthor.wordpress.com) I took a look at Pappenheimer's suggestions and the game itself.
Well the result was that I added the following stuff:
# increasing the random value of the cave border during play. Now the better you are the harder it gets
# Collision detection between bullets and the cave border
# Collision Avoidance between enemies and cave border
# Small Game Menu (Start, Exit)

You can read more about Alien's Cave here:
http://xarthor.wordpress.com/games/aliens-cave/

And here is the download link to the new version:
http://www.file-upload.net/download-1847658/AliensCave.zip.html

Highscore: 34 - I'm pretty bad!

Nice little game, Xarthor!

It's not that easy to get a high score, though. At least for me.

There were only two minor bugs, I encountered:
1) once in a while enemies get stuck in the walls
2) sometimes the laser bolts fired by the player vanish before hitting anything (I think this happens when the player is in "invincible mode" after getting killed)

Game was fun nevertheless. : )