First flash game, feedback please

My first Flash game in now in a fully playable state (minus sound) and I would appreciate any feedback:

http://lab.wx3.com/defender/

Screenshot:

I wrote it as an exercise to learn ActionScript/Flash, and as a first program, I’m fairly happy with the results. It’s a “tower defense” style game where the player must defend Earth from various approaching alien craft. The graphics are mostly done using bitmap manipulation, which seems to be a bit of a departure from the more common vector style graphics in these games, but allow for some (I think) cool effects.

Any responses would be great, but particularly I’m interested in:
How does it run on your machine?
What other features do you feel it needs to be a successful game?
Is the interface intuitive?
Most importantly, is it fun?

Thanks a lot!