It's good for a first
Okay, it's definitely not the most advanced or best game out there. But it is a first try, and I'm willing to go a little easier on you for that. Here's a couple of tips, from a non-flash programmer's (just gamer) perspective...
-Most TD games use a variety of towers, so if you make another try to incorporate that into it. It's nice if they're all upgradeable too... I'm sure you've played other ones, so try and be like that, basically.
-Make the shooting of the towers automatic. Not only does it save our fingers, it adds strategy.
-Lastly, design a complicated path to add more elements of strategy.
Okay, I basically just listed some stuff you probably already knew... But hopefully it can help you make a kickass game next time!