Hmm... sorry, as far as I can tell that should work. By the way, if the cities remain stationary and the missiles are moving around the screen, it might make more sense to draw the cities to a collision layer not the missiles, and compare the missiles to it. Then you only need to clear and redraw the city layer when a city is destroyed rather than every frame, which I think could make it faster.
However, I might have no idea what I'm talking about.
However, I might have no idea what I'm talking about.