You have a limited amount laser power to wipe stop the assorted nasties overunning your base. You also have another problem a limited amount of time. To shoot the enemy you wait until they are in your line of fire. Using a combination of the keys given below, you can conserve your laser power and shoot as many aliens as possible.
Use ‘space' to fire in all directions. Use ‘Z' to fire horizontally and ‘backslash' to fire vertically.
The time and number of shots could be altered to make the game easier or harder. There could be some random harmless obstacles placed on the screen as decoys to infuriate the player. A final suggestion would be to change the colours of the display as the time increases and have a warning noise to indicate impending doom.
Amstrad Games Book