Tower Defenses are all about juggling resources, and in this case, there's only one resource to juggle. And by gosh, it is a juggle! When the game picks up and you have to keep a bunch of different plates spinning, it really gets good, and it's satisfying to beat back attacks on five different fronts at once.
Two suggestions I'd have:
1) Make it a little more clear what the tower attack types do. I think the Fire one was slow combined with a damage multiplier, but because the units' health bars appeared I thought it was dealing damage over time. In the same way, it was difficult to tell that the knights were immune to bombs, since their health bar appeared when hit.
2) At the end, the mortar enemy appears just a little too quickly. If I invested the time to take him out, a new one would spawn immediately, and it felt like I'd just wasted my time for no progress.
All in all, a wonderful game, and a creative take on the one-tower Tower Defense!