Very well done and polished gameplay!
There haven't been as many tower defense games as I expected when the theme was announced, or at least I haven't found that many of them myself. This one works very nicely, I especially liked setting the second chapter at night for some graphical variety.
The gameplay is simple, as is expected of something put together in the time limit, but it works perfectly well as it is. I did become overconfident the first time I played and lost on round 9 when I had money to spare. The second time I fared better.
One note on this, surely it makes a bit more sense to phrase it as "[T]hey are not much for us!"? :D