Almost There
The concept is great; customizing a defense game layout. The execution, not so much. The choice of towers were a bit odd, and they lacked any kind of excitement. The enemies never grew stronger, so upgrading anything seemed a bit pointless. And the upgrades, meh. But peaking of which, since you seem to be the innovative type, how about (if a next installment is planned) adding an upgrade system that has sort of a rock-paper-scissors theme? Or multiple upgrade paths like in that one Pinic Ant Defense game? There is definitely some great potential here.