This game fails on the most crucial of points. It's not fun.
I'm on the second level and I'm already thinking "When is this going to end".
The length of the levels are too long for the little variety they show. It's just boring.
The upgrades don't offer anything remotely interesting like actual tower upgrades, which means you actively have to bombard all the creeps. It's tedious.
As for the look of the game, it's quite nice and the premise is good.
The execution though is a failure.
Better luck next time.