I really like the concept of this game, it reminds me of a game I made for LD once, but with a lot more focus on the tower defense. It started very well but it did get a bit boring later on as you can just build as many towers as possible and farm for more, I was expecting the tumbleweeds to get tougher somehow...
As one tidbit, the Flee button didn't work for me. I did like it very much, still.