Solid game...with only some flaws.
OK, I'm going to be deadly blunt here, so please don't burst into tears! :p
Music sucked. It was terrible and boring. There were big design flaws, like how you can't see your HP when your deciding whether to repair your base, because that unecessarily, overly-large box that holds the buttons covers the most important part of the HUD. Also, moving your mouse over to the '?' to see your power/HP/time left is hugely annoying when you have 30 guys coming in and shooting shit at you. Why not just make it semi-transparent and put small, neat little bars in the top left corner? It wouldn't obscure too much of the view.
Anyways, that was all of the negative stuff. The game itself kicked ass, and of course, pretty much the only thing I wanted more (just like in any of these sit-and-shoot games) was more powerups and more buildings to build onto my base. But I don't expect you to have thousands, I'm just saying it would be a nice feature if you make a second one.
You had great actionscript! I didn't find any bugs, and it was really smooth. The difficulty was perfect to keep me hooked by being challenging on some levels, but then laying off occassionally and reminding me that ultimately, I have more fun when I'm kicking ass. I liked that a lot and thought you designed that well into your game. I liked your graphics, although I thought you could have spent a little more time on them, but they really don't matter for a game of this genre. Also, the sound effects were shit, but I understand and I must say I was impressed at that whole game only being just under 2MB. I see why the sound was shit, but I would have waited longer for it to load to have better SFX.
Basically I think you should add on to it, or better yet make a sequel. I hope I've given enough suggestions as to what you should alter/change, and I thoroughly look forward to seeing a second one. Keep up the great scripting! 5/5.