Homework Tower Defence Certainly good enough.
One of the first things that struck me was that the graphics are pretty stiff. After a while, that doesn't matter that much, but a little animation on the "towers" wouldn't be so bad. I don't know if that's easy or hard though, so no more complaining there.
The gameplay - well, it's a Tower Defense game. Nothing more, nothing less. That part of it is well done. There is one thing here, though. I think it's too easy (basically because it's the first TD game I managed to finish without losing any lives). I mean, once you get the best of the towers, it's pretty much straight on.
I don't have anything else to add. I didn't have access to music, so I can't review that. Overall, the game was good with some replay value. Oh, and a rather different theme that many can relate to. Good job.