You're at the start of something good here, but you've got some pretty major issues to work through if you want to end up with an app that people will download, enjoy, and play.
First, for what you did well: The engine works pretty well. I had a moment or two of slowdown, but for a tower defense game, the gameplay usually doesn't move that fast, so I think you're okay there. The enemy movement, speed, and attack animations all did exactly what they were supposed to, and the fast forward button was a nice touch- that's a big step in game development.
I did have one major glitch- when my device auto-locked as I waited for the next wave (or maybe I hit the button, I don't remember), it started over from the title screen, and my game was lost- it didn't save.
Now for the constructive criticism:
The game has some serious balance issues. I found the cheapest units to be 100% worthless- using all my money on them didn't even stop the third wave, much less anything beyond that. This means that if you don't start with the $20 unit, you're pretty much dead right at the start. If you do start with them, though, you still might lose a unit or two until you can get a few built up- at which point the game becomes a breeze, as it's just wave after wave of enemies.
You really should find an artist at your school to redo some of the art. Some of it looked pretty good, but there were a lot of places where the sprites had some leftover pixels along the edge. You had a lot of green sprites on green background, too, making them very hard to see.
As for the programming, you really need health bars and level progress indicators. Check out Robo Defense- they do a lot of this well. Having something for users to do while the game is going on helps a lot, too- check out Plants Vs Zombies for a great example of how this can work.
Please don't be discouraged by all the criticism, though- you've got a good start here, and it's got the potential to be something great.