Kobo Deluxe is an enhanced version of Akira Higuchi's game XKobo for Un*x systems with X11. Kobo Deluxe adds sound, smoother animation, high resolution support, OpenGL acceleration (optional), an intuitive menu driven user interface, joystick support and other features. Recent versions also add a number of alternative skill levels with slightly modernized gameplay.
The objective is to destroy all the structures on each level by shooting the purple bulb in the center of the structure. Of course, various enemy craft are trying to get you at the same time. This game is strangely addictive.