Use the arrow keys to move the scratch cat. Avoid black bombs and the "red baron", as well as the red bar that appears periodically. The green spheres give you an extra life. Press the space for a temporary shield. You get more shields as time goes on. Get extra lives by running the "red baron" into your shield. The game gets progressively harder, and eventually reaches the level of unplayable (unless, of course, you find the cheats).
I hope to polish the appearance, but for now you can still have fun. Thanks to Mr. Herzog, who showed me Scratch. Thanks, sir.