Not bad, but not good.
I like the layout of the defense maps. Some maps are much more challenging then others, like the split map with two bases or the little fortress where enemies come from all sides.
My problems with the game is how fast the health and frequency of the enemies increase. The payout isn't large enough to upgrade successfully. I have tried many different strategies and they all seemed to fail because of this. I recommend making the payout larger so the player can upgrade faster.