Spectral Invaders

Earth is under attack from fleets of alien space ships. Conveniently lined up in nice grids, the aliens sweep side to side, dropping one row each time they reach the edge of the screen. Avoiding their bombs, the player has a tank that can trundle left and right at the bottom of the screen, hiding behind gradually crumbling buildings for cover, and has to shoot the aliens before they can land. Yes, that's right, it's a version of Space Invaders for the Spectrum.