Pretty interesting concept - I don't think I've seen a "reverse tower defense" type of game before, and I think it would be cool to see a more fleshed out version of this type of game. I think adding some instructions would be a good idea, because I wasn't really sure what to do at first. Once I figured out that I could click the button to make something happen, I wasn't sure what the goal was (especially since this game kind of a reversal of the typical tower defense).