Swoop is yet another cross-version of Space Invaders/Galaxians, but better than most of the versions about. Rolling two games into one always seems a good idea, but very rarely pays off.
In Swoop, you have to shoot the aliens who move across the screen Invaders-style. Then one breaks off and "swoops" down to bombard your base. If the alien misses, it plants an 'egg' which restricts the movement of your base as you cannot cross over these eggs. After a short time, the eggs explode.
Although not one of the most exciting games, it is certainly hard, and good value for money.