Star Castle:

You control a spaceship in a field of stars. In the center of the field is a round fortress composed of three spinning force fields, inside is a large space cannon. The ship has a similar feel to the one in in Asteroids, but it is a bit larger.

To defeat the cannon you must blast your way through the rings and shoot the cannon itself. That is easier said than done because a new set of rings appears each time you blast an old set completely away. So you have to try and preserve a bit of each ring, and hope for a lucky shot at the cannon, which will probably fire at you as well if it has a clear shot.

The cannon isn't your only enemy. Three indistinct "sparks" begin on the rings, but soon break away to pursue your ship, they are easily blasted, but they will return again and again.

You would score 10 points for each outer ring section you blasted, 20 points for each center ring section, and 30 points for each inner ring section. Blowing away the cannon itself would score you an extra life. This game does not seem to have a maximum score like most early games, but a bug slows the game down a bit after 182,000 points.

Early versions of this game allowed for almost unlimited play by flying a pattern of going off the right side of the screen, then the top, and rotating to fire a few shots on each pass. A later revision to the game fixed this "bug" by speeding up the sparks slightly.