I like the idea of turning the theme into a tower defense game. Right now it has some improvements to do though. It is hard to figure out where and how to build things. Enemies spawn rather quickly, so you usually end up losing a lot before you know what to do. I didn't manage to get past the second level either, the enemies just went through the obstacles and I couldn't inflict enough damage. I probably needed to use some other obstacle, but in the end I gave up.
The game would benefit much from some sort of in-game tutorial level.