The game Castle Blast is based on the fifth screen of the coin-op game Phoenix. The object of this game is to destroy the Castle on the Mothership. You fire Vertical Trajectory Missiles at the shield on the bottom of the Mothership. When you penetrate the shield, then blast the Castle. To score extra points, you can also shoot the enemy spaceships flying overhead.
The gameplay involves using your joystick to move left or right and the lower button to fire Vertical Trajectory Missiles. The upper button fires Horizontal Trajectory Missiles, which are needed for ground-based tank assaults in later levels. As you progress through the game, new enemies and obstacles appear, making it increasingly difficult to destroy the Castle.