Great, original idea.
I never thought I'd see a game in which the point was to GET attacked. What's that tank made of? I mean, for something to perpetually get hit by bombs at still be unscratched would probably a high enough mass and density to exert a major gravitational force on Earth it runs on. Imagine putting the entire mass of the himalayas in a golf ball. It would probably have to be made of that stuff. :)